Acha Mountain Fortress is an earthen 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 Mount Acha, 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. Acha Mountain Fortress was designated as a historic landmark in 1973, and has since become a common local tourist destination.