Early uses tomb brick showing gate towers Defensive fences for protecting humans and domestic animals against predators was used long before the appearance of writing and began "perhaps with primitive man blocking the entrances of his
caves for security from large
carnivores". From very early history to modern times,
walls have been a necessity for many cities.
Amnya Fort in western Siberia has been described by archeologists as one of the oldest known fortified settlements, as well as the northernmost Stone Age fort. In Bulgaria, near the town of
Provadia a walled fortified settlement today called
Solnitsata starting from 4700 BC had a diameter of about , was home to 350 people living in two-story houses, and was encircled by a fortified wall. The huge walls around the settlement, which were built very tall and with stone blocks which are high and thick, make it one of the earliest walled settlements in Europe but it is younger than the walled town of
Sesklo in Greece from 6800 BC.
Uruk in ancient
Sumer (
Mesopotamia) is one of the world's oldest known
walled cities. The Ancient Egyptians also built fortresses on the frontiers of the
Nile Valley to protect against invaders from adjacent territories, as well as circle-shaped mud brick walls around their cities. Many of the fortifications of the ancient world were built with mud brick, often leaving them no more than mounds of dirt for today's archeologists. A massive prehistoric stone wall surrounded the ancient temple of
Ness of Brodgar 3200 BC in
Scotland. Named the "Great Wall of Brodgar" it was thick and tall. The wall had some symbolic or ritualistic function. The
Assyrians deployed large labor forces to build new
palaces, temples and defensive walls.
Bronze Age Europe , Malta. Borġ in-Nadur is a notable example of
Bronze Age fortifications. In
Bronze Age Malta, some settlements also began to be fortified. The most notable surviving example is
Borġ in-Nadur, where a bastion built in around 1500 BC was found. Exceptions were few—notably, ancient
Sparta and ancient
Rome did not have walls for a long time, choosing to rely on their militaries for defense instead. Initially, these fortifications were simple constructions of wood and earth, which were later replaced by mixed constructions of stones piled on top of each other without
mortar. In
ancient Greece, large stone walls had been built in
Mycenaean Greece, such as the ancient site of
Mycenae (famous for the huge stone blocks of its '
cyclopean' walls). In
classical era Greece, the city of
Athens built two parallel stone walls, called the
Long Walls, that reached their fortified seaport at
Piraeus a few miles away. In
Central Europe, the
Celts built large fortified settlements known as
oppida, whose walls seem partially influenced by those built in the
Mediterranean. The fortifications were continuously being expanded and improved. Around 600 BC, in
Heuneburg, Germany, forts were constructed with a limestone foundation supported by a
mudbrick wall approximately 4 meters tall, probably topped by a roofed walkway, thus reaching a total height of 6 meters. The wall was clad with lime plaster, regularly renewed. Towers protruded outwards from it. , a
Gaulish oppidum, showing the construction technique known as
murus gallicus.
Oppida were large fortified settlements used during the
Iron Age. The
Oppidum of Manching (German: Oppidum von Manching) was a large Celtic proto-urban or city-like settlement at modern-day Manching (near Ingolstadt), Bavaria (Germany). The settlement was founded in the 3rd century BC and existed until . It reached its largest extent during the late La Tène period (late 2nd century BC), when it had a size of 380 hectares. At that time, 5,000 to 10,000 people lived within its 7.2 km long walls. The oppidum of
Bibracte is another example of a Gaulish fortified settlement.
Bronze and Iron Age Near East . The term
casemate wall is used in the archeology of
Israel and the wider
Near East, having the meaning of a double wall protecting a city or fortress, with transverse walls separating the space between the walls into chambers. Casemate walls became a common type of fortification in the Southern Levant between the Middle Bronze Age (MB) and Iron Age II, being more numerous during the Iron Age and peaking in Iron Age II (10th–6th century BC). Casemate walls could surround an entire settlement, but most only protected part of it. The three different types included freestanding casemate walls, then integrated ones where the inner wall was part of the outer buildings of the settlement, and finally filled casemate walls, where the rooms between the walls were filled with soil right away, allowing for a quick, but nevertheless stable construction of particularly high walls.
Ancient Rome near
Porta San Sebastiano The
Romans fortified their cities with massive, mortar-bound stone walls. The most famous of these are the largely extant
Aurelian Walls of
Rome and the
Theodosian Walls of
Constantinople, together with partial remains elsewhere. These are mostly city gates, like the
Porta Nigra in
Trier or
Newport Arch in
Lincoln.
Hadrian's Wall was built by the
Roman Empire across the width of what is now
northern England following a visit by
Roman Emperor Hadrian (AD 76–138) in AD 122.
Indian subcontinent , Gujarat 2600 BCE A number of forts dating from the
Later Stone Age to the
British Raj are found in the mainland
Indian subcontinent (modern day
India,
Pakistan,
Bangladesh and
Nepal). "Fort" is the word used in India for all old fortifications. Numerous
Indus Valley Civilization sites exhibit evidence of fortifications. By about 3500 BC, hundreds of small farming villages dotted the
Indus floodplain. Many of these settlements had fortifications and planned streets. The stone and mud brick houses of
Kot Diji were clustered behind massive stone flood dykes and defensive walls, for neighboring communities bickered constantly about the control of prime agricultural land. The fortification varies by site. While
Dholavira has stone-built fortification walls,
Harrapa is fortified using baked bricks; sites such as
Kalibangan exhibit
mudbrick fortifications with bastions and
Lothal has a quadrangular fortified layout. Evidence also suggested of fortifications in
Mohenjo-daro. Even a small town—for instance, Kotada Bhadli, exhibiting sophisticated fortification-like bastions—shows that nearly all major and minor towns of the Indus Valley Civilization were fortified. Forts also appeared in urban cities of the Gangetic valley during the second urbanization period between 600 and 200 BC, and as many as 15 fortification sites have been identified by archeologists throughout the Gangetic valley, such as
Kaushambi,
Mahasthangarh,
Pataliputra,
Mathura,
Ahichchhatra,
Rajgir, and
Lauria Nandangarh. The earliest
Mauryan period brick fortification occurs in one of the stupa mounds of Lauria Nandangarh, which is 1.6 km in perimeter and oval in plan and encloses a habitation area.
Mundigak () in present-day south-east
Afghanistan has defensive walls and square
bastions of sun dried bricks. ,
Rajasthan, India India currently has over 180 forts, with the state of
Maharashtra alone having over 70 forts, which are also known as
durg, many of them built by
Shivaji, founder of the
Maratha Empire. A large majority of forts in India are in North India. The most notable forts are the
Red Fort at
Old Delhi, the
Red Fort at
Agra, the
Chittor Fort and
Mehrangarh Fort in
Rajasthan, the
Ranthambhor Fort,
Amer Fort and
Jaisalmer Fort also in Rajasthan and
Gwalior Fort in
Madhya Pradesh.
Arthashastra, the Indian treatise on military strategy describes six major types of forts differentiated by their major modes of defenses.
Sri Lanka built by King
Kashyapa I of Anuradhapura. Forts in
Sri Lanka date back thousands of years, with many being built by Sri Lankan kings. These include several walled cities. With the outset of colonial rule in the
Indian Ocean, Sri Lanka was occupied by several major colonial empires that from time to time became the dominant power in the Indian Ocean. The colonists built several western-style forts, mostly in and around the coast of the island. The first to build colonial forts in Sri Lanka were the
Portuguese; these forts were captured and later expanded by the
Dutch. The
British occupied these Dutch forts during the
Napoleonic wars. Most of the colonial forts were garrisoned up until the early 20th century. The coastal forts had
coastal artillery manned by the
Ceylon Garrison Artillery during the two world wars. Most of these were abandoned by the military but retained civil administrative officers, while others retained military garrisons, which were more administrative than operational. Some were reoccupied by military units with the escalation of the
Sri Lankan Civil War;
Jaffna fort, for example, came under siege several times.
China near
Jinshanling. The Great Wall was a series of fortifications built across the historical northern borders of China. Large tempered earth (i.e.
rammed earth) walls were built in
ancient China since the
Shang dynasty (–1050 BC); the capital at ancient Ao had enormous walls built in this fashion (see
siege for more info). Although stone walls were built in China during the
Warring States (481–221 BC), mass conversion to stone architecture did not begin in earnest until the
Tang dynasty (618–907 AD). The
Great Wall of China had been built since the
Qin dynasty (221–207 BC), although its present form was mostly an engineering feat and remodeling from the
Ming dynasty (1368–1644 AD). In addition to the Great Wall, a number of Chinese cities also constructed
defensive walls to defend their cities. Notable
Chinese city walls include the city walls of
Hangzhou,
Nanking, the
Old City of Shanghai,
Suzhou,
Xi'an and the
walled villages of Hong Kong. The famous walls of the
Forbidden City in
Beijing were established in the early 15th century by the
Yongle Emperor. The Forbidden City made up the inner portion of the
Beijing city fortifications.
North America Frontier forts In the United States, there are many examples of historical forts or
fortified "stations" - the name depended on the region. This was a structure built for defense against primarily
Indian attacks in frontier areas. While some forts were sometimes used by
militias, state and federal military units, their primary purpose was for private or civilian defense. Sometimes a
stockade would surround the building(s). Examples of historic private or civilian fortified houses built include: •
Fort Nelson and
Floyd's Station and
Low Dutch Station all in
Kentucky •
Mormon Fort and
Mormon Station in
Nevada •
Fort Buenaventura,
Cove Fort,
Fort Deseret, and
Fort Utah all in
Utah. •
Carpenter's Fort in
Ohio •
Fort Bigham,
Fort Swatara, the
Heinrich Zeller House, and
Fort Depuy, 18th-century fortified homesteads in Pennsylvania
Philippines Spanish colonial fortifications During the
Spanish Era several forts and outposts were built throughout the archipelago. Most notable is
Intramuros, the old
walled city of Manila located along the southern bank of the
Pasig River. The historic city was home to centuries-old churches, schools, convents, government buildings and residences, the best collection of Spanish colonial architecture before much of it was destroyed by the bombs of
World War II. Of all the buildings within the 67-acre city, only one building, the San Agustin Church, survived the war. Partial listing of Spanish forts: •
Intramuros, Manila •
Cuartel de Santo Domingo, Santa Rosa, Laguna •
Fuerza de Cuyo, Cuyo, Palawan •
Fuerza de Cagayancillo, Cagayancillo, Palawan •
Real Fuerza de Nuestra Señora del Pilar de Zaragoza, Zamboanga City •
Fuerza de San Felipe, Cavite City •
Fuerza de San Pedro, Cebu •
Fuerte de la Concepcion y del Triunfo, Ozamiz, Misamis Occidental •
Fuerza de San Antonio Abad, Manila •
Fuerza de Pikit, Pikit, Cotabato • Fuerza de Santiago,
Romblon, Romblon • Fuerza de Jolo,
Jolo, Sulu • Fuerza de Masbate,
Masbate • Fuerza de Bongabong,
Bongabong, Oriental Mindoro • Cotta de Dapitan,
Dapitan, Zamboanga del Norte • Fuerte de Alfonso XII,
Tukuran, Zamboanga del Sur • Fuerza de Bacolod,
Bacolod, Lanao del Norte • Guinsiliban Watchtower,
Guinsiliban, Camiguin • Laguindingan Watchtower,
Laguindingan, Misamis Oriental • Kutang San Diego,
Gumaca, Quezon • Baluarte Luna,
Luna, La Union Local fortifications The Ivatan people of the northern islands of Batanes built their so-called
idjang on hills and elevated areas to protect themselves during times of war. These fortifications were likened to European castles because of their purpose. Usually, the only entrance to the castles would be via a rope ladder that would only be lowered for the villagers and could be kept away when invaders arrived. , 1898. Fort Santiago was a
citadel that was a part of the
Intramuros, a walled city within Manila. The
Igorots built forts made of stone walls that averaged several meters in width and about two to three times the width in height around 2000 BC. The Muslim Filipinos of the south built strong
fortresses called
kota or
moong to protect their communities. Usually, many of the occupants of these kotas are entire families rather than just warriors. Lords often had their own kotas to assert their right to rule, it served not only as a military installation but as a palace for the local Lord. It is said that at the height of the
Maguindanao Sultanate's power, they blanketed the areas around Western
Mindanao with kotas and other fortifications to block the Spanish advance into the region. These kotas were usually made of stone and bamboo or other light materials and surrounded by trench networks. As a result, some of these kotas were burned easily or destroyed. With further Spanish campaigns in the region, the sultanate was subdued and a majority of kotas dismantled or destroyed. kotas were not only used by the Muslims as defense against Spaniards and other foreigners, renegades and rebels also built fortifications in defiance of other chiefs in the area. During the American occupation, rebels built strongholds and the datus, rajahs, or sultans often built and reinforced their kotas in a desperate bid to maintain rule over their subjects and their land. Many of these forts were also destroyed by American expeditions, as a result, very very few kotas still stand to this day. Notable kotas: •
Kota Selurong: an outpost of the
Bruneian Empire in Luzon, later became the City of
Manila. •
Kuta Wato/Kota Bato: Literally translates to "stone fort" the first known stone fortification in the country, its ruins exist as the "Kutawato Cave Complex" •
Kota Sug/Jolo: The capital and seat of the
Sultanate of Sulu. When it was occupied by the Spaniards in the 1870s they converted the kota into the world's smallest walled city.
Pre-Islamic Arabia During Muhammad's lifetime , 627. Muslim defenders repelled the Confederates using Medina's natural fortifications and makeshift trenches. During
Muhammad's era in Arabia, many tribes made use of fortifications. In the
Battle of the Trench, the largely outnumbered defenders of Medina, mainly
Muslims led by
Islamic prophet Muhammad, dug a
trench, which together with Medina's natural fortifications, rendered the confederate
cavalry (consisting of horses and
camels) useless, locking the two sides in a stalemate. Hoping to make several attacks at once, the confederates persuaded the Medina-allied
Banu Qurayza to attack the city from the south. However,
Muhammad's diplomacy derailed the negotiations, and broke up the confederacy against him. The well-organized defenders, the sinking of confederate morale, and poor weather conditions caused the siege to end in a fiasco. During the
Siege of Ta'if in January 630, Muhammad ordered his followers to attack enemies who fled from the
Battle of Hunayn and sought refuge in the fortress of Taif.
Islamic world Africa The entire city of
Kerma in
Nubia (present day Sudan) was encompassed by fortified walls surrounded by a ditch. Archeology has revealed various Bronze Age bastions and foundations constructed of stone together with either baked or unfired brick. The
walls of Benin are described as the world's second longest man-made structure, as well as the most extensive earthwork in the world, by the
Guinness Book of Records, 1974. The walls may have been constructed between the thirteenth and mid-fifteenth century CE or, during the first millennium CE. Strong citadels were also built other in areas of Africa.
Yorubaland for example had several sites surrounded by the full range of earthworks and ramparts seen elsewhere, and sited on ground. This improved defensive potential—such as hills and ridges. Yoruba fortifications were often protected with a double wall of trenches and ramparts, and in the Congo forests concealed
ditches and paths, along with the main works, often bristled with rows of sharpened stakes. Inner defenses were laid out to blunt an enemy penetration with a maze of defensive walls allowing for entrapment and
crossfire on opposing forces. of
Kankelefa (1849) Between the 15th and 19th centuries, forts and walled cities were constructed on a large scale from the
Senegal River to settlements of the
Niger River. The types of fortification ranged from the
Tata to the
Ribat. Stone castles and fortified villages were evident in Ethiopia since the 13th century. In
Southern Africa, urban centres were walled since the 13th century with notable examples at
Mapungubwe and
Great Zimbabwe. A military tactic of the
Ashanti was to create powerful log
stockades at key points. This was employed in later wars against the
British to block British advances. Some of these fortifications were over a hundred yards long, with heavy parallel tree trunks. They were impervious to destruction by artillery fire. Behind these stockades, numerous Ashanti soldiers were mobilized to check enemy movement. While formidable in construction, many of these strongpoints failed because Ashanti guns, gunpowder and bullets were poor, and provided little sustained killing power in defense. Time and time again British troops overcame or bypassed the stockades by mounting old-fashioned bayonet charges, after laying down some covering fire.
Medieval Europe , Poland, made of field stone and
bog iron.
Roman forts and
hill forts were the main antecedents of
castles in
Europe, which emerged in the 9th century in the
Carolingian Empire. The
Early Middle Ages saw the creation of some towns built around castles. These cities were only rarely protected by simple stone walls and more usually by a combination of both walls and
ditches. From the 12th century, hundreds of settlements of all sizes were founded all across Europe, which very often obtained the right of fortification soon afterward. 's 1624 map of
Bermuda with the first stone fortifications built by
England in the
New World. The founding of urban centers was an important means of territorial expansion and many cities, especially in
eastern Europe, were founded precisely for this purpose during the period of
Ostsiedlung. These cities are easy to recognize due to their regular layout and large market spaces. The fortifications of these settlements were continuously improved to reflect the current level of military development. During the
Renaissance era, the
Venetian Republic raised great walls around cities, and the finest examples, among others, are in
Nicosia (Cyprus),
Rocca di Manerba del Garda (Lombardy), and
Palmanova (Italy), or
Dubrovnik (Croatia), which proved to be futile against attacks but still stand to this day. Unlike the Venetians, the
Ottomans used to build smaller fortifications but in greater numbers, and only rarely fortified entire settlements such as
Počitelj,
Vratnik, and
Jajce in
Bosnia.
Development after introduction of firearms Medieval-style fortifications were largely made obsolete by the arrival of
cannons on the 14th century
battlefield. Fortifications in the age of
black powder evolved into much lower structures with greater use of
ditches and
earth ramparts that would absorb and disperse the energy of cannon fire. Walls exposed to direct cannon fire were very vulnerable, so were sunk into ditches fronted by earth slopes. This placed a heavy emphasis on the
geometry of the fortification to allow defensive cannonry interlocking fields of fire to cover all approaches to the lower and thus more vulnerable walls. , 1728. The development of bastion forts resulted from the increased use of cannons and firearms in the 14th century. The evolution of this new style of fortification can be seen in transitional forts such as
Sarzanello in North West Italy which was built between 1492 and 1502. Sarzanello consists of both crenellated walls with towers typical of the medieval period but also has a
ravelin like angular gun platform screening one of the curtain walls which is protected from flanking fire from the towers of the main part of the fort. Another example is the
fortifications of Rhodes which were
frozen in 1522 so that Rhodes is the only European walled town that still shows the transition between the classical medieval fortification and the modern ones. A manual about the construction of fortification was published by
Giovanni Battista Zanchi in 1554. Fortifications also extended in depth, with protected batteries for defensive cannonry, to allow them to engage attacking cannons to keep them at a distance and prevent them from bearing directly on the vulnerable walls. , a
sea fortress from 18th century in
Helsinki, Finland The result was
star shaped fortifications with tier upon tier of hornworks and
bastions, of which
Fort Bourtange is an excellent example. There are also extensive fortifications from this era in the
Nordic states and in
Britain, the fortifications of
Berwick-upon-Tweed and the harbor
archipelago of
Suomenlinna at
Helsinki being fine examples.
19th century During the 18th century, it was found that the continuous
enceinte, or main defensive enclosure of a bastion fortress, could not be made large enough to accommodate the enormous field armies which were increasingly being employed in Europe; neither could the defenses be constructed far enough away from the fortress town to protect the inhabitants from bombardment by the besiegers, the range of whose guns was steadily increasing as better manufactured weapons were introduced. Therefore, since refortifying the
Prussian fortress cities of
Koblenz and
Köln after 1815, the principle of the
ring fortress or
girdle fortress was used: forts, each several hundred meters out from the original enceinte, were carefully sited so as to make best use of the terrain and to be capable of mutual support with neighboring forts. Gone were citadels surrounding towns: forts were to be moved some distance away from cities to keep the enemy at a distance so their artillery could not bombard said urbanized settlements. From now on a ring of forts were to be built at a spacing that would allow them to effectively cover the intervals between them. The arrival of
explosive shells in the 19th century led to yet another stage in the evolution of fortification.
Star forts did not fare well against the effects of high explosives and the intricate arrangements of bastions, flanking batteries and the carefully constructed lines of fire for the defending cannon could be rapidly disrupted by explosive shells. of
Fort Delimara. Built in 1878, Delimara was built as a typical
polygonal fort ditches and counterscarps made to be very deep, vertically sided, and cut directly into the rocks. Worse, the large open ditches surrounding forts of this type were an integral part of the defensive scheme, as was the covered way at the edge of the
counterscarp. The ditch was extremely vulnerable to bombardment with explosive shells. In response, military engineers evolved the
polygonal style of fortification. The ditch became deep and vertically sided, cut directly into the native rock or soil, laid out as a series of straight lines creating the central fortified area that gives this style of fortification its name. Wide enough to be an impassable barrier for attacking troops but narrow enough to be a difficult target for enemy shellfire, the ditch was swept by fire from defensive
blockhouses set in the ditch as well as firing positions cut into the outer face of the ditch itself. The profile of the fort became very low indeed, surrounded outside the ditch covered by
caponiers by a gently sloping open area so as to eliminate possible cover for enemy forces, while the fort itself provided a minimal target for enemy fire. The entrypoint became a sunken gatehouse in the inner face of the ditch, reached by a curving ramp that gave access to the gate via a rolling bridge that could be withdrawn into the gatehouse. , German fortifications built in 1893. By the 19th century, tunnels were used to connect
blockhouses and firing points in the ditch to the fort. Much of the fort moved underground. Deep passages and
tunnel networks now connected the
blockhouses and firing points in the ditch to the fort proper, with
magazines and machine rooms deep under the surface. The guns, however, were often mounted in open emplacements and protected only by a
parapet; both in order to keep a lower profile and also because experience with guns in closed
casemates had seen them put out of action by rubble as their own casemates were collapsed around them. The new forts abandoned the principle of the bastion, which had also been made obsolete by advances in arms. The outline was a much-simplified polygon, surrounded by a ditch. These forts, built in masonry and shaped stone, were designed to shelter their garrison against bombardment. One organizing feature of the new system involved the construction of two defensive curtains: an outer line of forts, backed by an inner ring or line at critical points of terrain or junctions (see, for example,
Séré de Rivières system in France). Traditional fortification however continued to be applied by European armies engaged in warfare in colonies established in Africa against lightly armed attackers from amongst the indigenous population. A relatively small number of defenders in a fort impervious to primitive weaponry could hold out against high odds, the only constraint being the supply of ammunition.
20th and 21st centuries , built in the 1930s. Due to the threat of
aerial warfare, the buildings were placed at a distance from each other, making it difficult to find from the air.
Steel-and-
concrete fortifications were common during the 19th and early 20th centuries. However, the advances in modern warfare since
World War I have made large-scale fortifications
obsolete in most situations. In the 1930s and 1940s, some fortifications were built with designs taking into consideration the new threat of
aerial warfare, such as
Fort Campbell in Malta. Despite this, only underground
bunkers are still able to provide some protection in modern wars. Many historical fortifications were demolished during the modern age, but a considerable number survive as popular tourist destinations and prominent local
landmarks today. The downfall of permanent fortifications had two causes: • The ever-escalating power, speed, and reach of artillery and airpower meant that almost any target that could be located could be destroyed if sufficient force were massed against it. As such, the more resources a defender devoted to reinforcing a fortification, the more combat power that fortification justified being devoted to destroying it, if the fortification's destruction was demanded by an attacker's strategy. From
World War II,
bunker busters were used against fortifications. By 1950,
nuclear weapons were capable of destroying entire cities and producing dangerous
radiation. This led to the creation of civilian nuclear
air raid shelters. • The second weakness of permanent fortification was its very permanency. Because of this, it was often easier to go around a fortification and, with the rise of mobile warfare in the beginning of World War II, this became a viable offensive choice. When a defensive line was too extensive to be entirely bypassed, massive offensive might could be massed against one part of the line allowing a breakthrough, after which the rest of the line could be bypassed. Such was the fate of the many defensive lines built before and during World War II, such as the
Siegfried Line, the
Stalin Line, and the
Atlantic Wall. This was not the case with the
Maginot Line; it was designed to force the Germans to invade other countries (Belgium or Switzerland) to go around it, and was successful in that sense. bomb hits the ground. The development of
bunker busters, bombs designed to penetrate hardened targets buried underground, led to a decline in the use of fortifications. Instead field fortification rose to dominate defensive action. Unlike the
trench warfare which dominated World War I, these defenses were more temporary in nature. This was an advantage because since it was less extensive it formed a less obvious target for enemy force to be directed against. If sufficient power were massed against one point to penetrate it, the forces based there could be withdrawn and the line could be reestablished relatively quickly. Instead of a supposedly impenetrable defensive line, such fortifications emphasized
defense in depth, so that as defenders were forced to pull back or were overrun, the lines of defenders behind them could take over the defense. Because the mobile offensives practiced by both sides usually focused on avoiding the strongest points of a
defensive line, these defenses were usually relatively thin and spread along the length of a line. The defense was usually not equally strong throughout, however. The strength of the defensive line in an area varied according to how rapidly an attacking force could progress in the terrain that was being defended—both the terrain the defensive line was built on and the ground behind it that an attacker might hope to break out into. This was both for reasons of the strategic value of the ground, and its defensive value. This was possible because while offensive tactics were focused on mobility, so were defensive tactics. The dug-in defenses consisted primarily of infantry and
antitank guns. Defending
tanks and
tank destroyers would be concentrated in mobile
brigades behind the defensive line. If a major offensive was launched against a point in the line, mobile reinforcements would be sent to reinforce that part of the line that was in danger of failing. Thus the defensive line could be relatively thin because the bulk of the fighting power of the defenders was not concentrated in the line itself but rather in the mobile reserves. A notable exception to this rule was seen in the defensive lines at the
Battle of Kursk during World War II, where German forces deliberately attacked the strongest part of the
Soviet defenses, seeking to crush them utterly. The
terrain that was being defended was of primary importance because
open terrain that tanks could move over quickly made possible rapid advances into the defenders' rear areas that were very dangerous to the defenders. Thus such terrain had to be defended at all costs. is an underground
bunker used by
North American Aerospace Defense Command. Cheyenne Mountain is an example of a mid-20th century fortification built deep in a mountain. In addition, since in theory the defensive line only had to hold out long enough for mobile reserves to reinforce it, terrain that did not permit rapid advance could be held more weakly because the enemy's advance into it would be slower, giving the defenders more time to reinforce that point in the line. For example, the
Battle of the Hurtgen Forest in Germany during the closing stages of World War II is an excellent example of how difficult terrain could be used to the defenders' advantage. After World War II,
intercontinental ballistic missiles capable of reaching much of the way around the world were developed, so speed became an essential characteristic of the strongest militaries and defenses.
Missile silos were developed, so missiles could be fired from the middle of a country and hit cities and targets in another country, and airplanes (and
aircraft carriers) became major defenses and offensive weapons (leading to an expansion of the use of airports and airstrips as fortifications). Mobile defenses could be had underwater, too, in the form of
ballistic missile submarines capable of firing
submarine launched ballistic missiles. Some bunkers in the mid to late 20th century came to be buried deep inside mountains and prominent rocks, such as
Gibraltar and the
Cheyenne Mountain Complex. On the ground itself,
minefields have been used as hidden defenses in modern warfare, often remaining long after the wars that produced them have ended.
Demilitarized zones along borders are arguably another type of fortification, although a passive kind, providing a buffer between potentially hostile militaries.
Military airfields Military airfields offer a fixed "target rich" environment for even relatively small enemy forces, using
hit-and-run tactics by ground forces, stand-off attacks (mortars and rockets), air attacks, or ballistic missiles. Key targets—aircraft, munitions, fuel, and vital technical personnel—can be protected by fortifications. Aircraft can be protected by
revetments,
hesco barriers,
hardened aircraft shelters and
underground hangars which will protect from many types of attack. Larger aircraft types tend to be based outside the operational theater. Munition storage follows safety rules which use fortifications (bunkers and bunds) to provide protection against accident and chain reactions (sympathetic detonations). Weapons for rearming aircraft can be stored in small fortified
expense stores closer to the aircraft. At Biên Hòa, South Vietnam, on the morning of May 16, 1965, as aircraft were being refueled and armed, a chain reaction explosion destroyed 13 aircraft, killed 34 personnel, and injured over 100; this, along with damage and losses of aircraft to enemy attack (by both
infiltration and stand-off attacks), led to the construction of revetments and shelters to protect aircraft throughout
South Vietnam. Aircrew and ground personnel will need protection during enemy attacks and fortifications range from culvert section "duck and cover" shelters to permanent air raid shelters. Soft locations with high personnel densities such as accommodation and messing facilities can have limited protection by placing prefabricated concrete walls or barriers around them, examples of barriers are
Jersey Barriers, T Barriers or Splinter Protection Units (SPUs). Older fortification may prove useful such as the old 'Yugo' pyramid shelters built in the 1980s which were used by US personnel on 8 Jan 2020 when Iran fired 11 ballistic missiles at
Ayn al-Asad Airbase in Iraq. Fuel is volatile and has to comply with rules for storage which provide protection against accidents. Fuel in underground bulk fuel installations is well protected though valves and controls are vulnerable to enemy action. Above-ground tanks can be susceptible to attack. Ground support equipment will need to be protected by fortifications to be usable after an enemy attack. Permanent (concrete) guard fortifications are safer, stronger, last longer and are more cost-effective than
sandbag fortifications. Prefabricated positions can be made from concrete culvert sections. The British Yarnold Bunker is made from sections of a concrete pipe. Guard towers provide an increased field of view but a lower level of protection. Dispersal and camouflage of assets can supplement fortifications against some forms of airfield attack.
Counterinsurgency Just as in colonial periods, comparatively obsolete fortifications are still used for low intensity conflicts. Such fortifications range in size from small patrol bases or
forward operating bases up to huge
airbases such as
Camp Bastion/
Leatherneck in
Afghanistan. Much like in the 18th and 19th century, because the enemy is not a powerful military force with the heavy weaponry required to destroy fortifications, walls of
gabion,
sandbag or even simple mud can provide protection against small arms and antitank weapons—although such fortifications are still vulnerable to mortar and artillery fire. ==Forts==