Gongsanseong (공산성) does not rank high on the 'most seen by foreigners' list. Then again, neither does the city of Gongju. Naturally, the Lady in Red and I had to check it out.
Originally called Ungjinseong, the mud fortress was used by King Munjuwang (reigned 475-477 A.D.) as the Baekje capital during the Baekje Dynasty. In 538 A.D., however, King Seong moved the capital to present-day Buyeo, although Gongju remained fairly important (and close to the capital) until the Baekje Dynasty collasped in 667 A.D.
The fortress was rebuilt with stones during the Joseon Dynasty, and King Injo hid here for a short time as well.