The original walls, built in the late 14th century, were constructed of medium-sized round stones held together by mud. During King
Sejong the Great's reign in the mid-15th century, a large-scale refurbishment work was carried out on the wall, including the replacement of earthen wall sections with rectangular stone sections. A major restoration in 1704 by King
Sukjong rebuilt sections of the wall using large, uniform stone slabs which mark the final and last unique characteristic of Hanyangdoseong. The eastern section of Seoul was on lower ground than the other sections and was more susceptible to external attack. Thus, a
lookout was added to the outside of the gate to reinforce its defense. A part of the walls in the section between
Heunginjimun and
Gwanghuimun was extended outside in a rectangular shape for such a purpose. Beacon mounds, another component of the defense system, were first established in 1394 and remained in operation until 1894. Signals sent across the country from one mound to another, using smoke by day and fire at night, were received by the beacon at the top of
Namsan and conveyed to the Royal Palace. One lit beacon indicated normalcy, two indicated the appearance of an enemy, three if the enemy approached the border, four if the enemy crossed the border and five in case a battle started at the border.
Gates Four main gates and four auxiliary gates were built around Seoul in the late 14th century. The four main gates were
Dongdaemun (East Gate),
Seodaemun (West Gate),
Namdaemun (South Gate) and
Sukjeongmun (North Gate). The four auxiliary gates were placed in areas between the four main gates, with
Souimun (in the southwest),
Changuimun (in the northwest),
Hyehwamun (in the northeast) and
Gwanghuimun (in the southeast). At present, the following gates are either preserved in their original form or have undergone restoration work: Sungnyemun (South Gate) and Heunginjimun (East Gate) are designated as National Treasure No. 1 and Treasure No. 1, respectively. ==Present condition==