Great Wall, fortification along the northern and northwestern frontier of China. The largest portion of the wall was erected by Shihuangdi (Shih-huang-ti), first emperor of the Qin (Ch'in) dynasty, as a defense against raids by nomadic peoples. Systematic work on the wall was begun about 221 BC, and it was finished about 204 BC. In succeeding centuries, chiefly during the period of the Ming dynasty (AD 1368-1644), the Great Wall was repaired and extended. The fortification finally reached a length of about 2400 km (about 1500 mi). The wall is built of earth and stone, faced with brick in the eastern parts. It averages about 6 m (about 20 ft) thick at the base. The height averages 7.6 m (25 ft).