The immense structure, built as a barricade of protection against invasion, was begun during the third century B.C. by Emperor Shih Huang Ti (Cheng; c. 259–210 B.C.) of the Ch’in dynasty, and was expanded over the course of succeeding centuries. The wall stretches 1,500 miles, ranges in height between 20 to 50 feet, and is between 15 and 25 feet thick. In the thirteenth century, the wall was penetrated when Mongols conquered China, expanding their empire across all of Asia.