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. Con­struc­tion con­tin­ued up to the Ming dynasty (1368–1644), when the Great Wall became the world‘s largest mil­i­tary struc­ture. Its his­toric and strate­gic impor­tance is matched only by its archi­tec­tur­al sig­nif­i­cance.