Tourists visit the Zhonghuamen Gate, a barbican that is part of the ancient city wall of Nanjing, capital of east China's Jiangsu Province, Aug. 8, 2014. Nanjing's city wall was constructed between 1366 and 1386, the early years of the Ming Dynasty (1368-1644). Once the dynasty's capital, Nanjing used to be enclosed by four layers of stone-brick city walls. After six centuries of natural erosion and human destruction on purpose, the length of the remaining city wall totals 25 kilometers today, which still makes a world record. (Xinhua)