The Great Wall of China was built from 220-206 B.C. It was rebuilt, maintained, and enhanced many times by the Ming Dynasty. The main reason for building this wall was for protecting themselves from the Mongolians and other groups and tribes.The people built the wall on top of forests and mountains. That gave them an advantage.
Your answer is that the wall was built in 220-206 B.C.
Construction of the Great Wall of China was begun during the third century B.C. by Emperor Shih Huang-ti (c. 259–210 B.C.) of the Ch'in dynasty (family of rulers). The immense structure was built as a protective barricade against invasion and was expanded over many centuries. The wall now stretches 1,500 miles, ranging in heights between 20 and 50 feet (6.1 and 15 meters), and is 15 to 25 feet (4.6 to 7.6 meters) thick. Guard stations and watchtowers are placed at regular intervals, but successive invasions of China from the north have proved the wall to be fairly ineffectual. Since 1949 a section of the wall in North Peking has been reconstructed and opened to visitors.
Further Information: Fisher, Leonard Everett. The Great Wall of China. New York: Macmillan, 1986; Great Wall of China. [Online] Available http://www.china-tours.com/, October 23, 2000; Lo, Che-Wen. The Great Wall. New York: McGraw-Hill, 1981; McNeese, Tim. The Great Wall of China. San Diego: Lucent Books, 1997.