This is a fortress built of the stones found easily around the boundary of this valley. Originally, it was made fortress of the Baekje Dynasty made with piled up of earth. A citation, from the Samguk sagi (History of the Three Kingdoms): “The construction of a walled city named Seowonsulseong by order of a general, the third son of Kim Yu-sin (a contributing commander-in-chief of the Silla for winning victory in the wars among the three kingdoms). . .”, has some experts and scholars conjecture the building period of the Sangdangsanseong to be the early years of the Unified Silla period. Sangdang is an ancient name of the present-day Cheongju city, and this region was called Sangdang from the time of the Baekje Dynasty. This fortress had two times experience of reconstruction work in the Joseon Dynasty, one at the time of ending the war against Japan that broke out in 1592 and the other in 1716, the 42nd year of the reign of King Sukjong. Granite blocks were used in the construction, and the fortress wall remained in good condition but a low fence on the top of the wall has disappeared There would have been five ponds, three Buddhist temples, a government office building and a warehouse in the fortress and now, still, a gate and a low wall remain. The top of the fortress commands a good view, in particularly in the westerly direction, indicating that the political and military purpose of the fortress at that time was the defense of this region from enemy attack. This has been a position of strategic importance to the national defense since the Baekje Dynasty, and especially in the Joseon Dynasty.