The Ancient City Wall of Xi'an (also known as the Xi'an Ming City Wall) is the largest, oldest, and most completely preserved military fortification system in China. Originally constructed in 1370 AD during the Ming Dynasty, it was built atop the foundations of the earlier Tang Dynasty imperial city. The massive wall stands 12 meters (39 feet) tall, stretches 13.74 kilometers (8.5 miles) in a perfect rectangular loop, and functions as a geographic boundary separating historic central Xi'an from the modern expanding metropolis.