The park in Beijing has the longest and best preserved section of the city’s Ming Dynasty city wall. The park is located 3 km from the city center and extends west from Chongwenmen to Dongbianmen and then north to near the Beijing Railway Station. The park features a 1.5 km section of the Ming city wall and the Southeast Corner Tower, which are over 550 years old and surrounded by green park space to the south and east.

Pictures taken on 6 April 2014.
Beijing’s inner city wall was built during the Ming Dynasty in 1419. The Ming city walls stood for nearly 550 years until the construction of the early 1960s when most of the gates and walls were torn down to construct the Beijing Subway, which runs underneath where the walls stood. The subway’s inner loop line turned into the old city at Chongwenmen to stop at the Beijing Railway Station, and did not need to run beneath a section of the wall at the southeast corner of the inner city. Of the 40 km of the original wall, only this 1.5 km section was spared. Inside this section of the wall (north of the wall) are railway yards of Beijing Station. Outside the wall (south of the wall) stood residential homes and small businesses.
In the late 1990s, the city government decided convert the remnants of the wall into a park and relocate the small businesses and homes between the foot of the southern city wall and Chongwenmen East Avenue. Construction began in November 2001 and the park was completed in September 2003.
More details: http://en.wikipedia.org/wiki/Ming_City_Wall_Relics_Park