Beijing used to be surrounded by a 25km long city wall. The wall was torn down in the 50s and replaced more or less with the current Second Ring Road. A remnant piece of the city wall 1.5km long is now made into a Beijing Ming City Wall Ruins Park.
The southeast watch-tower was built in 1436 and is the largest remaining city wall watch-tower in China.
From Gladys Garden, take subway line 8 at Shichahai station and go north one stop to Gulou Dajie station, change to line 2 and ride 6 stops to Jianguomen, Exit C. Then you walk south for 1100m.