The existing city wall of Xi’an is an extension of the prior Tang dynasty structure, it represent one of the oldest and best preserved Chinese city walls. It was started by the Ming Dynasty in 1370 and finished in 1378. With a rectangular–shaped construction, the city wall measures 13.7 kilometers (8.5 mi) in circumference, 12 meters (39 ft.) in height, and 15–18 meters (49–59 ft.) in thickness at the base. There is a rampart every 120 meters on the top of the city wall, which extends out from the main wall. In 1961, the city wall of Xi’an was included as a key heritage site under national protection by the State Council of China. On 28 March 2008, China proposed the city wall of Xi’an be included on UNESCO's World Heritage List, where it is categorized as a tentative listing.