This is a fortress located around Achasan Mountain (285 meters above sea level), Gwangjin-gu, Seoul, and was built of stone during the Three Kingdoms Period. The total length of this fortress is 1,125 meters and the height is about 10 meters. There remain traces of gates and waterways on the east, the west, and the south, and also the curved walls which were built to protect the inner wall by covering the front of gate. Many pieces of earthenware and broken roof tiles were found at various sites. It is told that Achasanseong Fortress has 2 tragic histories. One is that the King Gaero of the Baekje Dynasty was killed under the walls when Hansan, the capital city of Baekje, was occupied by Goguryeo forces, the other is that in the fortress there is the Ondalsaem spring which has kept a historical tradition saying that General Ondal, the son-in-law of King Pyeongwon of Goguryeo, died under the walls in a battle with Silla forces to recover the lost southern area between Goguryeo and Jungnyeong.
In 1997, the Achasanseong Fortress was excavated by a team from Seoul National University Museum, and, after many relics including more than 100 pieces of earthenware, more than 30 pieces of steel artifacts, and more than 100 pieces of weapons were discovered, it was confirmed that it is a Goguryeo, not a Baekje site. This work is expected to contribute greatly to the study of the culture of the Goguryeo Dynasty around this region, though further detailed investigation and excavation are required. Achasanseong Fortress has significance as a historical site where the Three Kingdoms competed to occupy the valley of the Hangang River, though there still remains debate as to by which dynasty it was built.