The World Heritage–listed fortress of Namhansanseong (남한산성 도립공원), 20km south-east of central Seoul, once guarded the city’s southern entrance. Today it's famous for hiking trails which hug the 17th-century fortress walls, of which 12.3km still remain, taking you through beautiful pine and oak forests, and wild flowers.
Your first stop should be Namhansanseong Emergency Palace, the beautifully reconstructed complex of the king's quarters, which also has a hiking map of the fortress.
The most popular hiking route is the two-hour loop that leads you past the main gates of Bukmun (North Fortress), Seomun (West), Nammun (South) and South Command Post, with sweeping panoramas. Or you can trek the entire wall's perimeter in around seven hours. Be sure to mix up trails that lead in and out of the wall to change your views.
To get here, take subway Line 8 to Sanseong, then get a taxi or take bus 9 from Exit 2 of the station to the park’s south gate, a total journey of around one hour from central Seoul.