Unlike so many of China's modernized but ancient cities, Xi'an has managed to retain a major piece of its past intact. Forming an enormous rectangle around the old imperial heart of the city, the Xi'an city walls date back to the Tang Dynasty (618-907 AD), though they attained their present brick-faced form in 1568 and several sections have been recently rebuilt. The most complete city wall in China still in existence, they run approximately 12 kilometers in length and feature imposing guard towers and gates. In their heyday, the city was further protected by a moat complete with drawbridges; today, the moat has been filled in and converted to park land. The wall is fully walkable, with access provided at the major gates, making it a great way to get a feel for the lay of the land. A shuttle runs between the South, East and West Gates and bikes can be rented for RMB 20 an hour for riding the wall.