The Great Wall was a gigantic defensive project used in ancient times as early as in the 7th century B.C. For self-protection, rival kingdoms built walls around their territories, laying foundations for the present Great Wall. When Qin Shihuang (First Emperor of the Qin) unified the whole country in 221 B.C., the existing walls were linked up and new ones added to counter attacks by the remnants of the defeated states. The undertaking of such a huge project over difficult terrain at that time without any machinery was an extraordinary feat. A workforce of nearly a million, representing one fifth of the whole labour force of the country, was used to build it. In its prime, the Wall was close to 4,000 miles long.