Mangsan Fortress is known as the foremost defensive castle of Daegaya where they fought the Shilla invaders. During the Japanese Invasion of 1592, the loyal troops rebuilt the castle to fight the Japanese invaders. It is about 1km to the east of Downtown Goryeong on Mt. Geum at the boundary of Goryeong-eup, Gaejin-myeon, and Seongsan-myeon.

The main building stretches longitudinally, facing the topography to the west that rises 290 above sea level. It is made of both stone and earth and is about 600m in circumference. The walls have collapsed, but the west side still forms a stiff cliff. The east side is forming a stair-like cliff that is relatively moderate. There are many pieces of the walls scattered along the longitudinal ridgeline.

The fortress still has a beacon. Also, there are more than 100 small stone plate dolmens from Gaya along the southern ridgeline.