Achasanseongseong Fortress boasts magnificent views of the Hangang River and skyline. The 200m-high mountain fortress wall was built to face southeast, towards the Hangang River. It is also called Janghan-seong or Gwangjang-seong. A fierce battle was once fought here among Gogury, Baekje and Silla because of its strategic location during the period of the Three Kingdoms. King Gae-Ro of Baekje (r. 455-475) died in the war by the Goguryeo forces sent by King Jang-Su (r. 413-491) and General Ondal of Goguryeo was also killed in a battle with the Silla army in 590.

Achasan Mountain is popular as a citizens’ resting place with a historical trail and a natural park including various facilities such as walkway, mountain walkway, a badminton court, a wrestling range, an archery range, various amusement facilities for children, benches, and pavilion.