Construction of the Great Wall began during the Spring and Autumn Period to the Warring States Period (770BC-221BC), Several ducal states built wall fortifications along the precipitous mountain ridges to ward off  the plunder of nomadic people and the invasion of each other. In 221 BC, Qin Shi Huang ordered to link the walls up after he united China.(full Stroy)

 

 

 

From:  http://www.ccnt.com.cn/culture/relic/newcnh/changch/changch01-e.htm