The City Walls were built in 1370 and have recently been restored. The walls are about 12m high and 18m wide at base and are built as a rectangle around the center of Xian. The total length is 12km.
To explore the walls, you can rent a bike or a small golf cart, or you can hop on a small tourist bus. This photo was taken from the top floor of one of the guard towers (similar to the ones you see on the photo). Notice the people in the middle of the photo. You can rent an "emperor" costume to feel like the emperor for a few minutes...