a system of fortified walls with a roadway along the top, constructed as a defense for China against the nomads of the regions that are now Mongolia and Manchuria: completed in the 3rd century b.c., but later repeatedly modified and rebuilt. 2000 miles (3220 km) long.
British Dictionary definitions for great wall china
Great Wall of China
a defensive wall in N China, extending from W Gansu to the Gulf of Liaodong: constructed in the 3rd century bc as a defence against the Mongols; substantially rebuilt in the 15th century. Length: over 2400 km (1500 miles). Average height: 6 m (20 ft). Average width: 6 m (20 ft)