Unjusanseong Fortress, also known as Gosansanseong Fortress, is a cultural heritage of the Baekje dynasty. This is a mountain fortress built along the surrounding three mountain peaks in the westernmost and southernmost parts of Unjusan Mountain. The stone fortress wall, which borders three villages in Jeondong-myeon and two villages in Jeonui-myeon, is 3,210 meters long and two meters wide with a x_height that ranges from two to eight meters. The walls follow the natural geographical features of the mountain range, with the southern walls surrounding the mountain's peak.

Remnants of a village can still be observed within the fortress walls, such as the fortress gate and a ritual site. Pieces of earthenware from the Baekje period, porcelain from the Goryeo and Joseon periods, and broken tiles from the Baekje, Goryeo and Joseon periods have been discovered around the site as well.