A group of about ten fortresses at Achasan Mountain (State-designated Cultural Heritage Historic Site No. 455) (designated in October 2004) is located longitudinally in Achasan and Yongmasan or along the Jungrangcheon Stream and Highway 3. These small structures, each with a perimeter of about 100m-300m, have been dated back to the Three Kingdoms Period based on the construction methods and relics discovered there. They are important materials which provide firm evidence of Goguryeo’s occupation of the areas along the Hangang River in the late 5th century and their eventual capture by Silla and Baekje in 551.

The fortresses constitute the largest single group of sites associated with the Goguryeo Kingdom in South Korea. They provide clues to the status of Goguryeo’s military facilities and other relevant archaeological facts. The fortresses at Achasan Mountain designated as historic sites include two at Hongryeonbong Peak, five at Achasan, six at Yongmasan, one at Sirubong Peak, one at Mangusan, and one at Suraksan. Those located in Jungrang-gu include three at Yongmasan and one at Mangusan.