Achasanseong Fortress is an earthen mountain fortress of the Korean Three Kingdoms period. Originally built by Baekje, it was occupied in turn by each of the Three Kingdoms. It stands on Achasan Mountain, in Gwangjin-gu, Seoul, South Korea, at an altitude of 200 meters above sea level. It has a perimeter of roughly 1 kilometer and an area of about 3375 m².