Seoul consistently ranks amongst the top five largest cities in the world by population and physically, it's freaking huge.
Well within the confines of what is modern north Seoul runs a defensive wall of significant size (between 5-8m high) called Hanyangdoseong or in English the Seoul City Wall, an original name huh?
It was originally built in 1396 and runs for 18.6km from Changuimun to Hyehwamun in an inverted U shape with Seoul’s primary palaces in the centre of the walls confines.
The wall protects the northern rear of Gyeongbokgung; Seoul’s largest royal palace and the adjacent “Blue House” (Office of the President). The western section of the trail was only reopened to the public in 2007 following the infiltration of Seoul by North Korean special forces in 1968 following the Korean War armistice in 1953. Since the armistice North Korea has attempted to assassinate South Korea's President numerous times and the Koreans still aren’t taking any chances. To this day razor wire runs along some parts of the wall and photography either side of those sections is not permitted (this is enforced by guards/guides).
There are numerous entries and exits to the trail depending on where you start and finish the walk. We started our walk at Changuimum Gate in the North West section of the trail at the foot of Baegak Mountain (the militarised area) and walked to Hyehwamun Gate at the North East section. If you’re feeling particularly energetic you could keep going and walk around the entirety of old Seoul following the patchwork circular chain of city walls as far south as the base of Nasam Tower and from there heading north again to Inwangsan Mountain... but that’s for crazy people.
Keep an eye on the date and day of the week because many historic sites across Seoul are closed on Mondays and if a public holiday falls on a Monday the date of closure can change.
Finally bring your passport, ARC (Alien Registration Card) or National ID if you wish to walk over Baegak Mountain. Registration is compulsory and coloured lanyards are given out and must be worn. The colour dictates the direction in which you are walking.
Check out the following address for an excellent map of the entire network of mountain trails. Information is available both in Korean and English. The link provided lead to the English section of the site.