Great Wall of China
The Great Wall of China is a series of fortifications made of stone, brick, tamped earth, wood, and other materials, generally built along an east-to-west line across the historical northern borders of China in part to protect the Chinese Empire or its prototypical states against intrusions by various nomadic groups or military incursions by various warlike peoples or forces. Several walls had already been begun to be built beginning around the 7th century BC; these, later joined together and made bigger, stronger, and unified are now collectively referred to as the Great Wall. Especially famous is the wall built between 220–206 BC by the first Emperor of China, Qin Shi Huang. Little of that wall remains. Since then, the Great Wall has on and off been rebuilt, maintained, enhanced; the majority of the existing wall was reconstructed during the Ming Dynasty.
Other purposes of the Great Wall have included border controls, allowing the imposition of duties on goods transported along the Silk Road, regulation or encouragement of trade and the control of immigration and emigration. Furthermore, the defensive characteristics of the Great Wall were enhanced by the construction of watch towers, troop barracks, garrison stations, signaling capabilities through the means smoke or fire, and the fact that the path of the Great Wall also served as a transportation corridor.
The Great Wall stretches from Shanhaiguan in the east, to Lop Lake in the west, along an arc that roughly delineates the southern edge of Inner Mongolia. The most comprehensive archaeological survey, using advanced technologies, has concluded that all the walls measure 8,851.8 km (5,500.3 mi). This is made up of 6,259.6 km (3,889.5 mi) sections of actual wall, 359.7 km (223.5 mi) of trenches and 2,232.5 km (1,387.2 mi) of natural defensive barriers such as hills and rivers.