Located 25 kilometers southeast of Seoul, Namhansanseong is a mountain fortress from the Joseon Dynasty (1392~1910). Spanning 12 kilometers, the fortress served as a emergency temporary capital. It was first built in the early 7th century, but the fortress people see today was constructed in the 17th century during the reign of King Injo before being reconstructed several times until the 19th century. The structure reflects the Japanese and Chinese fortress construction techniques between the 16th and 18th centuries and compiles various defensive military skills aimed at responding to the inflow of Western-style weapons. Situated on a castle site from the Unified Silla era, the fortress has been enlarged and expanded repeatedly. People continue to inhabit the area to this day. The mountain fortress is a striking illustration of different development stages of fortress construction skills, as well as the changes in the weapons system from the 7th to 19th centuries.