Continuous defensive wall stretching from western Gansu to the Gulf of Liaodong (2,250 km/1,450 mi). It was once even longer. It was built under the Qin dynasty from 214
BC to prevent incursions by the Turkish and Mongol peoples and extended westwards by the Han dynasty. Some 8 m/25 ft high, it consists of a brick-faced wall of earth and stone, has a series of square watchtowers, and has been carefully restored. It is so large that it can be seen from space.
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