Sangdangsanseong was a mountain fortress built in Sanseong-dong during the Joseon Dynasty. It was built as a tongseong during the Three Kingdoms Period, and there is a theory that it was built as an earthen fortress during the Baekje Dynasty and then rebuilt by Kim Seo-hyeon, the third son of Silla general Kim Yu-sin, and another theory that it was built by Gung Ye. It is said that it occupied a very important position in the Korean Peninsula historically and played an important military role in defending the Hoseo region during the Joseon Dynasty. Sangdangsanseong had three gates in the east, west, and south and two secret gates.