What is "bukhansanseong"

The Bukhansanseong (, "fortress of the mountains north of the Han") is a fortress located in Gyeonggi-do and Seoul, South Korea, dating back to the middle Joseon period. The present fort was completed in 1711, though plans for the structure date back to 1659. The name is also given to a fortress mentioned in the Samguk Sagi, constructed by Gaeru of Baekje in 132 CE, and the two are often conflated although the putative connection is contested.

The modern Bukhansanseong was built to protect the approach to Seoul, filling a gap in Korea's defences that had become apparent during the second Manchu invasion of 1636 and the earlier Imjin War. The Bukhansanseong was used as a royal retreat in emergencies, and contains 120 rooms.