This is a fortress built of stone around the summit of Seongsan Mountain from where the Namhangang River can be seen. The name Ondalsanseong comes from a story about General Ondal, spoken in this region. He also was the son-in-law of King Pyeongwon of the Goguryeo Dynasty. There is a record saying it already existed under the name of Seongsanseong Fortress in the early Joseon Dynasty though it is not clear when it was built. It is a smaller scaled fortress with a girth of 683 meters, and built with regular piles of similar size stones on both sides. In the fortress some remains and surviving fountain sites were found, as well as a drainage way in the shape of a ladder at the outside of the wall. The building pattern of the southwestern gate and the projecting part of the eastern gate are remarkable and rare cases among ancient fortresses. There is a lime cave named Ondalgul under the cliff at the northeastern bank of the Namhangang River. Because the wall has been so well preserved, these ruins are important in the study of the way that fortresses were built.