In c. 220 B.C., under Qin Shi Huang, sections of earlier fortifications were joined together to form a united defence system against invasions from the north.

• 约公元前220年，一统天下的秦始皇将早起修建的一些断续防御工事连接成一个完整的防御系统，用以抵抗来自北方的侵略，在后来的一千多年里，长城的修建一直没有停止。