The Bukhan Sanseong (Hanja/English: 北漢山城/North Han Mountain Fortress) is a fortress located in Goyang DeogYang-gu, Gyeonggi Province (Bukhan-dong 1-1 beonji) and Seoul Eunpyeong-gu (31 Daeseomun Lane/Daeseomun-gil, JinGwan-dong 278-28 beonji), 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 King Gaeru of Baekje in 132 CE, and the two are often conflated although the putative connection is contested.
Bukhan Sanseong was a strategic military stronghold during Samguk Period when three states of Goguryeo, Baekje and Silla co-existed. The fortress was built by Baekje but the three states exerted every effort to conquer the place as means to conquer other countries. In the 19th year of King Gojong Wang Cheol (1232), Goryeo had fierce battle with Mongolia troops over the fortress.
King Woo, the 32nd King of Goryeo rebuilt the fortress. After the Japanese Imjin Invasion in 1592 and Byeongja Manchu Invasion of 1636, Joseon Dynasty decided total reconstruction of the fortress. The reconstruction was started at the order of King Sukjong in 1711 to transform the original fortress into one of 7620 stone steps.
The length of the fortress stretching around the steep ridge of the Mount Bukhan amounts to 12.7 kilometers and 14 gates were made. Since 1990, some gates including Daenammun, Daeseongmun, Daedongmun, Dongjangdae and walls of the defensive fortress of Hanyang have been repaired and restored. Around the mountain is Bukhan Sanseong which was a military stronghold to protect Hanyang from foreign invasion. The fortress is 12.7 kilometers long and 2 million pyeong wide (One pyeong is about 3.35 square meters). Since 1990, repairs have been made to restore parts of the fortress including Daenammun, Daedongmun, Daeseongmun, Bogungmun (보국문) and Dongjangdae.
The modern Bukhan Sanseong 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 Bukhan Sanseong was used as a royal retreat in emergencies, and contains 120 rooms.