Acha Mountain Fortress (아차산성, 阿且山城, acha.mountain.fortress) is an earthen fortress of the Three Kingdoms period. Originally built by Baekje, it was occupied in turn by each of the Three Kingdoms. It stands on Achasan, in Gwangjin-gu, Seoul, at an altitude of 200 meters above sea level. It has a perimeter of roughly 1 kilometer and an area of about 3375 m².
The first mention of this fortress in historical records comes from the Samguk Sagi's statement that King Chaekgye of Baekje ordered it to be fortified against an expected Goguryeo invasion. At that time, Acha Mountain Fortress would have played a key role in the defense of the Baekje capital Hanseong from the north.
In 475, Goguryeo overran this and the other defenses of Baekje, and Gaero of Baekje was brought to Acha and executed. The fortress and the rest of the Han River valley were conquered by Shilla in 553. The Goguryeo general Ondal later died here while trying to reclaim the fortress.
Acha Mountain Fortress was designated as a historic landmark in 1973, and has since become a common local tourist destination.