Xi'an's Ming Dynasty city walls are the best-preserved in China — 13.7km around, 12 metres high, wide enough to drive a car on top. You can rent bicycles at several points on the wall and cycle the full circuit in about two hours. The south gate (Nan Men) is the main entrance.
The walls date from 1370, built on the foundations of the old Tang Dynasty imperial city. They survived largely because they were in continuous military use right up until the 20th century. Today the top is paved and makes for one of the most memorable urban rides anywhere — you look down into the old city on one side and out toward the modern skyline on the other.
Go late in the afternoon if you can. The light is best, the heat is manageable, and if you time it right you will be up there at sunset. Evening visits are also worthwhile — the walls and gates are beautifully lit after dark.