The Great Wall is the only man-made structure on the earth visible from the moon. It was built during the Warring States period (476—221 B. C.) to prevent invasions by the nomadic nations from the north. It was completed by the first Qin emperor who united China into one country. The first Qin emperor mobilized several hundred thousand builders to link sections of the previous walls, thus completing what is known today as the Great Wall of China. The wall is 6,700 kilometers long from Shanhaikuan, which overlooks the gulf of Bo Hai, to Jiayuguan Pass in Gansu Province. The average height of the wall is 7.8 meters, and its width averages 6.5 meters at the base and 5.8 meters at the top. Rectangular lookout posts were built about every 100 meters.
The section of the Great Wall frequented by tourists is in Badaling, 75 kilometers to the North of Beijing.