Construction of the Great Wall started in the 7th century B.C. The vassal states under the Zhou Dynasty in the northern parts of the country each built their own walls for defence purposes. After the state of Qin unified China in 221 B.C., it joined the walls to hold off the invaders from the Xiongnu tribes in the north and extended them to more than 10,000 li or 5,000 kilometers. This is the origin of the name Of the "10,000-li Great Wall"
