Seongsan Fortress is located on Mt. Seongjae, Eubnaeri, Gimhwaeup, Cheorwon County. This is an advanced spot for defense and detection on the way to Cheorwon, Hwacheon, Hoeyang in the direction to Pyonggang. It's recorded as Seongsan Fortress in Sinjeung Donggukyeojiseunglam. It is said that 'the fortress was built with stones, 1,489 Korean feet round and 4 feet high.' After the Japanese Imperialism, the fortress was called Gadeung Fortress. The fortress is higher in the north and lower in the south. The whole length of the fortress wall is 982m, and the height is estimated at about 7m.

The walls of the fortress are mostly destroyed except for some parts; the southwestern wall is quite well intact. In particular, the door and some other places of the wall are well conserved with a height of 5m, and length of 50m. The stones of the wall were from natural rocks, and some traces of stone-cutting remain in the wall. On the other hand, the gate of the fortress remains at the southeastern gate area, and three other places such as the east wall and north wall still show traces of the gates. In the fortress, particularly in the north and the south areas, many Korean tiles are scattered, showing traces of some buildings.

Other facilities in the fortress included a well site, village shrine and quarry. Various types of relics such as a piece of tile, pieces of celadon porcelain and white porcelain, earthenware and a piece of chinaware were found at the fortress. Considering the building method and relics found at the fortress, it is estimated to have been built at least in the period of united Silla and has been renovated and used through the period the three kingdoms of Korea, Goryeo and Joseon Dynasties. Besides, Japanese building methods can be found in the fortress, which provides evidence of a Japanese soldier’s station at the fortress during the Japanese Invasion in Korea in 1592. The fortress has been relatively well preserved and has a significant academic value for research on fortresses and fortification methods.