Designated Historical Site No. 355. It is a Temoe type mountain fortress (fortress constructed around the summit of a mountain) at the summit of the Gyejok Mt. with land area of 73,987㎡ and circumference of approximately 1,200m at the elevation of 399m. The internal height of the existing fortress wall is 3.4m with external height of 7m, and the width of the upper portion is 3.7m. The height of the northern wall, which is best reserved is 10.5m while the height of the western wall is 6.8m. There are sites of gates with width of 4m on the east, west and south aspects of the fortress, and a site of a rectangular well with length of 110㎝, width of 75㎝ and height of 63㎝ with approximately 1m of waterway underneath the well.
There is a location at the summit that is presumed to have been smoke signal tower with remains of the structural site and cornerstone. It is situated at a strategic location in the downstream of Geum River, and is being presumed to be the Onsan Fortress during the Baekje Dynasty as a large number of earthenware of Baekje Dynasty has been excavated. It is said that, following the collapse of Baekje, Baekjebuheunggun interrupted the advancement of Silla forces with this fortress as the military base for short while, and it was used as the base for the Farmers’ Army during the Donghak Uprising at the end of Joseon Dynasty.
Although majority of the fortress wall is constructed of internally piled construction method using flat natural talc with dimensions of 7×30㎝, approximately 200m of the eastern wall was constructed with internal and externally interacted construction technique of piling up the construction material from inside and outside simultaneously. There remains cornerstone for door with hole with dimension of 12㎝ in diameter and 12㎝ in length outside the site of the southern gate.
As pieces of earthenware and ceramics of not only the Baekje Dynasty but also Silla, Goryeo and Joseon Dynasties are being excavated from the inner aspects of the fortress, evidencing that it is a mountain fortress that has been used continuously from the era of Three States to the era of Joseon Dynasty. This mountain fortress is characterized by the presence of bastion such as the Geonduseong below the fortress along with Chilhyeonseong, Neungseong, Naesajiseong, Usulseong, Jinhyeonseong and Sajeongseong ·in the nearby areas.