The Great Wall of China 

The Great Wall of China is a series of stone and earthen fortifications in northern China, built, rebuilt, and maintained between the 5th century BC and the 16th century to protect the northern borders of the Chinese Empire. Since the 5th century BC, several walls have been built that were referred to as the Great Wall. One of the most famous is the wall built between 220206 BC by the first Emperor of China, Qin Shi Huang. Little of that wall remains; the majority of the existing wall were built during the Ming Dynasty. The Great Wall stretches from Shanhaiguan in the east to Lop Nur in the west, along an arc that roughly delineates the southern edge of Inner Mongolia. The most comprehensive archaeological survey, using advanced technologies, has recently concluded that the entire Great Wall, with all of its branches, stretches for 8,851.8 km. This is made up of 6,259.6 km of sections of actual wall, 359.7 km of trenches and 2,232.5 km of natural defensive barriers such as hills and rivers. During the 1440s1460s, the Ming built an extension to the Great Wall, which is called "Liaodong Wall". This section of the Great Wall enclosed the agricultural heartland of the Liaodong province, protecting it against potential incursions by Jurched-Mongol Oriyanghan from the northwest and the Jianzhou Jurchens from the north. Towards the end of the Ming Dynasty, the Great Wall helped defend the empire against the Manchu invasions that began around 1600. Under the military command of Yuan Chonghuan, the Ming army held off the Manchus at the heavily fortified Shanhaiguan pass, preventing the Manchus from entering the Chinese heartland. The Manchus were finally able to cross the Great Wall in 1644, when the gates at Shanhaiguan were opened by Wu Sangui, a Ming border general who was bribed and blackmailed to do so and who also disliked the activities of rulers of the Shun Dynasty. The Manchus quickly seized Beijing, and defeated the newly founded Shun Dynasty and remaining Ming resistance, to establish the Qing Dynasty. Under Qing rule, China's borders extended beyond the walls and Mongolia was annexed into the empire, so construction and repairs on the Great Wall were discontinued.