Namhansanseong Fortress

A UNESCO World Heritage mountain fortress city 20 kilometers southeast of Seoul served as the emergency capital during the 1636 Manchu invasion and remains astonishingly intact, with 95% of its original walls still standing—outperforming Hwaseong Fortress in preservation integrity. The 11-kilometer circuit walk along the stone walls through oak forest with views over the Han River plain is the finest mountain fortress hike in Korea, taking about 4 hours to complete.