Great Wall of China China

Legendary

This 21,196 km wall is one of the New 7 Wonders of the World.

Photo by Keith Roper

The Great Wall of China is a series of fortifications that were built across the northern borders of ancient Chinese states and Imperial China as protection against nomadic groups from the Eurasian Steppe.

Several walls were built as early as the 7th century BC, with other stretches joined later by Qin Shi Huang (220–206 BC), the first emperor of China. Little of the Qin wall remains. Later, many successive dynasties built and maintained multiple stretches of border walls. The best-known sections of the wall were built by the Ming dynasty (1368–1644).

In addition to being used for defense, the Great Wall was also used for border controls, allowing duties to be put on goods transported along the Silk Road, regulation or encouragement of trade and the control of immigration and emigration. 

The walls defensive characteristics were enhanced by the construction of watchtowers, troop barracks, and garrison stations. Signaling capabilities were added through the use of smoke or fire. The path of the Great Wall also served as a transportation corridor.

The frontier walls built by different dynasties have multiple courses that they follow. Together, they stretch from Liaodong in the east to Lop Lake in the west, from the present-day Sino–Russian border in the north to Tao River (Taohe) in the south; along an arc that roughly delineates the edge of the Mongolian steppe; spanning 21,196.18 km (13,170.70 mi) in total.

Today, the defensive system of the Great Wall is generally recognized as one of the most impressive architectural feats in history.

Source: Wikipedia