The wall, which follows the ridges of Baegak(Bugaksan), Naksan(Naktasan), Namsan(Mongmyeoksan) and Inwangsan, the four main mountains surrounding the center of Seoul, has been rebuilt several times since its initial construction in 1396. The wall measures between 5 and 8m high and is 18.6km long and it served as city wall for the longest time among the existing walls in the world.
-http://seoulcitywall.seoul.go.kr/