After Qin Shi Huang unified China, he sent General Tian Meng to lead 300,000 troops and recruited hundreds of thousands of people to resist the invasion of the Huns in the north.
It took ten years to build the Great Wall from west Gansu to east Liaodong, across the entire northern Qin Dynasty, over 10,000 kilometers in the north.
When the Emperor Han Wu of the Han Dynasty, the Xiongnu invaded the Han Dynasty’s border, Qing Wei and Qubing Huo repelled the Xiongnu’s attack, then strengthened the old Great Wall of Qin Dynasty, and extended the length to the northwest to Yumenguan.
In the early Ming Dynasty, Ming Taizu, Yuanzhang Zhu sent general Da Xu to supervise and expand the Great Wall in order to prevent the Mongolian offensive. Xu not only repaired the original Great Wall, but also added many enemies, customs, and beacon towers along the line. Juyongguan, Yanmenguan, Shanhaiguan, were all built at this time.
The Badaling Great Wall north of Beijing was built at this time. The Great Wall was built for more than 200 years in the Ming Dynasty and finally completed the 12,700-mile Great Wall from east Yalu River to west Jiayuguan.