At the end of the 10th century Harold Bluetooth vied with tribal
Saxons, the
Holy Roman Empire—at that time governed by the (integrated) Saxon
Ottonian house—and the
Slavic Abodrit and
Veleti tribes for the control of the southern region of the
Jutland peninsula, This area was also the very same region presumed to have been the ancient homelands of the
Angles. Harald also had to contend with
Norse people exercising coastal raids. At the
Firth of Schlei lay
Hedeby, known in the contemporary literary sources as
Schleswig, where the
Danevirke complex of fortifications stretched across the foot of the peninsula, holding back the hostile hosts from entering the territory, as well as providing a safe trade route via
Ejderen from the
North Sea coastline into Hedeby and the
Baltic Sea. The entire complex of fortifications, bridges and roads, including
Ravninge Bridge on the land route towards
Jelling, are presumed to have been in the hands of Harold. The fortresses establish a string of strategic points stretching from Aggersborg at the north of Jutland southward across
Funen to end in Borgring at the east coast of
Zealand. They have been dated to the reign of Harold Bluetooth, who held sway until c. 985, where he was ousted by his son
Sweyn Forkbeard, who eventually conquered the
Anglo-Saxon kingdom of
Æthelred the Unready some years later. This is the apex of the
Jelling dynasty, its patristic line ending the year 1042 with
Harthacnut, son of
Cnut the Great, son of Sweyn. Fortifications of a similar shape and date have been found around other old towns in Scandinavia, including
Borgeby,
Trelleborgen, and
Helsingborg in
Skåne, Sweden, near
Aarhus in Jutland, and
Rygge,
Norway Other, similar forts across Northern Europe include the
Walcheren forts in
Friesland (built in the outskirts of the
Carolingian Empire, with a similar street orientation), and
Warham Camp in England, but these are not considered Viking ring forts as they were likely built at different periods in time. The precise purpose of the fortresses is unknown. Some historians argue that they functioned as military barracks or training grounds by
Sweyn Forkbeard. However, it is more likely that they were intended as defensive strongholds along strategic trade points and/or administrative outposts of the budding state. Søren Sindbæk has offered the hypothesis that the fortresses allowed local populations to seek shelter within the fortress walls against an enemy while waiting for assistance from friendly forces from afar; this means that the fortresses helped Harold Bluetooth to control vast territory and send his army to a particular part of his territory without worrying that the undefended parts would be conquered or plundered. Others have debated whether the fortresses were defensive structures, military strongholds, or primarily served as barracks, as well as the economic, religious, and symbolic significance of the fortresses. on the landscape, which at the time was obscured by farmland (it has subsequently been partially re-established) The fortresses were soon abandoned and never mentioned in the literary sources, effectively creating the enigma now known as
trelleborge (
Danish plural of 'trelleborg').
Nonnebakken was enveloped by the near contemporary municipal
bishopric of Odense, while the rest receded into the landscape. The modern discovery of these sites began in the 1930s, with the excavation of
Trelleborg in Denmark. Since then, a total of five sites have been officially accepted as Viking ring fortresses. During the royally funded research project
Kongens Borge (The Kings Castles), in 2010, Denmark applied for the admission of Trelleborg, Fyrkat and Aggersborg as
The Trelleborg Fortresses as a UNESCO
World Heritage cultural property, It is rumoured, that Denmark and Sweden, perhaps around that time, applied for admission of the Viking ring fortresses, but the records available at the World Heritage Centre cannot confirm that any Swedish ring fortresses were ever mentioned. The five
trelleborge presently known became the
Viking-Age Ring Fortresses World Heritage Site in 2023. == List of Viking ring fortresses ==