Seoul City Wall, or Hanyangdoseong, once traced the defensive edge of Joseon-era Seoul around the mountains and gates of the old capital. Substantial sections remain, and the eastern stretch near Dongdaemun is one of the easiest places to connect the wall with central Seoul's markets, palaces and traditional neighbourhoods.
Walk a section rather than treating it as a single point. The Hanyangdoseong Museum nearby gives useful context before climbing toward Naksan or following the stones around Dongdaemun.