Seoul Fortress is a valuable heritage that has gone through numerous repairs its erection in 1392 by King Taejo. Along with the establishment of the Joseon Dynasty, King Taejo has constructed the fortress to protect its capital city. After King Sukjong had extended it to Bukhansanseong Fortress in 1704, the Seoul Fortress Walls had spanned for 18.2 km. Unfortunately, from the 19th century to the present, parts of the wall that passed through the downtown area were taken down and only 10.5 km remain to date.
The section around Mount Bugaksan had been closed off to the public for 38 years until it was open on April 1, 2006. Located north of Seoul, this section of the fortress wall offers a superb view of downtown Seoul.
There is a simple procedure at the entrance. You must show identification, such as a passport, certificate of foreign registration, etc.
source: Korea Travel Guide