During the Warring States Period, the reason why Qin, Zhao and Yan built the Great Wall in the north was to defend the Xiongnu and Donghu from the south.

The Great Wall developed from cities and castles. As a military defense project, cities and fortresses began as early as the end of the primitive commune. Because it can play an effective defensive role, it has developed in general since it entered the class society. The Great Wall first appeared in the Spring and Autumn Period, such as Fangcheng of Chu State (in the present Nanyang area of Henan Province). During the Warring States Period, there were the Great Wall of Weixi County, the Great Wall of Zhangzhao Water, the Great Wall of Zhongshan West, the Great Wall of Yanyi Water and the Great Wall of Taishan Mountain. These Great Walls once played a great role in the war. The swift cavalry of nomadic people in the north is unpredictable, and no country can stop their attack and plunder, whether it is infantry or cavalry. Only by building the Great Wall and garrison defense can they be prevented from plundering south. Only by building the Great Wall in the north can we deploy the main forces in the annexation and defense war between countries and complete the great cause of reunification.