fortifications of Xi'an ancient capital of China , represent one of the oldest and best preserved Chinese city walls . Construction of the first city wall of Chang'an began in 194 BCE and lasted for four years. That wall measured 25.7 kilometres (16.0 mi) in length, 12-16 metres (39-52 ft) in thickness at the base. The area within the wall was c. 36 square kilometres (14 sq mi) . The existing wall was started by the Ming Dynasty in 1370. It encircles a much smaller city of 14 square kilometres (5.4 sq mi). The wall measures 13.7 kilometres (8.5 mi) in circumference, 12 metres (39 ft) in height, and 15-18 metres (49-59 ft) in thickness at the base.