Back when Beijing was 'Dadu', capital of Kublai Khan's Yuan dynasty, the city was sheltered by an earthwork wall, the northern battlements of which are marked by this 9km-long strip of sculpted parkland. Remarkably, traces of the old earthworks endure. From Beitucheng (literally 'north earthen wall') subway station cross to the south side of the moat and head east; in a few moments you'll spot the original, misshapen mound, overgrown with trees.
Dadu was positioned about 2km north of the current city, hence these walls are outside Beijing's subsequent Ming dynasty battlements (which were torn down in the 1960s and are now marked by the route of subway line 2 and the 2nd Ring Rd). Another notable spot in the park is a statue of Emperor Shizu, aka Kublai Khan, grandson of Genghis, whose Mongol hordes conquered China and established the Yuan dynasty in 1279. Nearby is a smaller statue of traveller Marco Polo, who was a trusted confidant of the khan.