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The Great Wall of China was built over 2,000 years ago, by Qin Shi Huangdi, the first emperor of China during the Qin (Ch'in) Dynasty (221 B.C - 206 B.C.). In Chinese the wall is called "Wan-Li Qang-Qeng" which means 10,000-Li Long Wall (10,000 Li = about 5,000 km).
After subjugating and uniting China from seven Warring States, the emperor connected and extended four old fortification walls along the north of China that originated about 700 B.C. (over 2500 years ago). Armies were stationed along the wall as a first line of defense against the invading nomadic Hsiung Nu tribes north of China (the Huns). Signal fires from the Wall provided early warning of an attack.
The Great Wall is one of the largest building construction projects ever completed. It stretches across the mountains of northern China, winding north and northwest of Beijing. It is constructed of masonry, rocks and packed-earth. It was over 5,000 km (=10,000 Li) long. Its thickness ranged from about 4.5 to 9 meters (15 to 30 feet) and was up to 7.5 meters (25 feet) tall.
During the Ming Dynasty (1368-1644), the Great Wall was enlarged to 6,400 kilometers (4,000 miles) and renovated over a 200 year period, with watch-towers and cannons added.
The Great Wall can be seen from Earth orbit, but, contrary to legend, is not visible from the moon, according to astronauts Neil Armstrong, Jim Lovell, and Jim Irwin.