A 21,000km dragon made of earth and rock stretches throughout the country of China. The historical story is as long and complicated as its structure.
The Great Wall was originally just clay walls, built by feudal countries during the Spring and Autumn War to fight aggression from the nomadic North and other countries.
When Qin Shihuang unified China in 221 BC, the Tibetan and Pacific plateau became a barrier to protect the mainland. But the Mongols, Turkey and Hungary could still invade from the northern mountains. In order to protect the territory, Qin Shihuang extended the clay wall, the precursor of the Great Wall later.
The Great Wall, China's longest defensive project, has fallen twice on the heels of the invasion of Mongol and Manchu troops.