The Great Wall of China is a series of stone and earthen fortifications in China, built, rebuilt, and maintained between the 5th century BC and the 16th century to protect the northern borders of the Chinese Empire from Xiongnu attacks during the rule of successive dynasties. Several walls, referred to as the Great Wall of China, were built since the 5th century BC. One of the most famous is the wall built between 220–206 BC by the first Emperor of China, Qin Shi Huang; little of it remains; it was much farther north than the current wall, which was built during the Ming Dynasty.
The Great Wall stretches over approximately 6,400 km (4,000 miles) from Shanhaiguan in the east to Lop Nur in the west, along an arc that roughly delineates the southern edge of Inner Mongolia, but stretches to over 6,700 km (4,160 miles) in total.At its peak, the Ming Wall was guarded by more than one million men. It has been estimated that somewhere in the range of 2 to 3 million Chinese died as part of the centuries-long project of building the wall.
It is hard to appreciate the Great Wall from space even a river was pointed out as the Great Wall:
The story published 11 May 2004 ‘Great Wall of China seen from space’ featured image interpretation errors. The feature that was claimed to be the Great Wall is actually a river running into the Miyun Reservoir, northeast of Beijing. Further acquisitions of the region are being planned with other ESA instruments to investigate the subject further.
Source: European Space Agency news release, May 11, 2004