Experts have given us the most credible answer. As early as the Warring States Periods (476 BC - 221 BC), the ruling powers began to build walls as a defense against the northern nomadic tribes and other potential enemies. The state of Chu (11th Century BC-223BC) was the first to build a wall, followed by the Qi, Yan, Wei, Zhao and Qin. After Emperor Qin Shihuang unified the six states, he ordered General Meng Tian to connect the existing walls and to extend them further as a front line defense against possible invasion. Thus the Great Wall of China was formed. It extended from Lintao (present Lintao County in Dingxi City in Gansu Province) in the west and ended at Liaodong (present the eastern and southern parts of Liaoning Province) in the east. It was called "Wan Li Chang Cheng" (The Long Wall of 10,000 Li).
Three groups of people built the wall. They were soldiers, common people and criminals. Many people died during its construction, due to the heavy work, short time limit and tough condition. An accurate number of those who died is unknown.
During subsequent dynasties, the wall was extended, repaired or modified. The section of wall built in Han Dynasty (206BC-220) is the longest. During the Ming Dynasty (1368-1644), the wall was substantially repaired on more than twenty occasions. Today, the best known and most visited sections of the wall are at Badaling, Mutianyu, Simatai and Jinshanling and these were built in the Ming Dynasty. They were all built to protect the people and territories south of the wall from a continued threat of invasion by the northern nomadic tribes.