Five College Center for East Asian Studies Study Tours Website
"Originally, this was the mountain fortress piled up of earth in the Baekje (one of three ancient and advanced kingdoms of Korean peninsula) Period.
Sangdang is an ancient name of Cheongju city at the present day and especially this region was called Sangdang from the time of the Baekje Kingdom.
This fortress had the experience of two times reconstruction works in the Joseon Kingdom: one is the time of after ending the war against Japan called Imjinwaeran broken out in 1592 and the other the 42nd year during the reign of King Sukjong (A.D. 1716) - particularly this was rebuilt of stones, new material again in the year." (Exploring Cultural Properties Website)
Wikipedia - Cheongju