Bukhansanseong, Hill castle in Gyeonggi Province, South Korea.

Bukhansanseong is a 12.7 km stone fortress completed in 1711 under King Sukjong, spanning 6.2 km² across the ridges of Bukhansan. Its walls link 6 great gates, 8 secret gates and 2 water gates, with three command posts and a temporary palace site inside. Sections along the peaks were built with only a parapet, without a full wall.

The fortress was built in 1711 under King Sukjong's rule, several years after invasions threatened the region. It served as a refuge and defensive stronghold during turbulent periods of Korean history.

The name comes from Bukhansan mountain, where Buddhist monks once served as soldiers within the walls. Their presence shaped the spiritual character of the site, which visitors can still sense while walking through the ruins.

The fortress lies inside Bukhansan National Park, spread across Goyang and several Seoul districts, so trails and gates connect from different valleys. The wall runs 12.7 km with 6 great gates, 8 secret gates and 2 water gates; plan a route rather than circling the whole circuit. Sections such as the Daeseomun gate and the Haenggung palace site are reached on foot over stone steps and uneven ground.

Hidden beneath the walls are wells and small reservoirs that allowed the fortress to survive months without external supply. These water systems reveal how carefully defenders planned for prolonged sieges.