Beijing - Great Wall of China

The Great Wall of China starts from Shanhaiguan Pass, a seaport along the coast of Bohai Bay in the east, to Jiayuguan Pass in Gansu Province in the west, covering 16 provinces, cities and autonomous regions in China. Its around 6400Km Long. It made me wonder why did they build a such a long wall? The chinese emperor wanted to protect their lands from the invading northern mongolians and huns.