The Great Wall

Designed as much to keep Chinese in as to keep Barbarians out of China, the Great Wall is considered one of the greatest human engineering feats of all time. Its overall length has never been full established but is well in excess of 8,850km (5,500 miles). Designed to stop raids, slow down invaders and warn the empire when and where major barbarian assaults had occurred, the wall protected the northern frontiers for several dynasties (Ch'in, T'ang and Ming making the greatest use of the Wall for this purpose). The wall also served to control trade, raise taxes via trade and limit who could enter and leave the Empire as citizens, merchants and visitors traveled through the various gateways and cities that were incorporated in the Wall.
The Great Wall was first completed by Qin Shi Hunagdi in the Second Century BCE. Over the next 1800 years the wall was strengthened or allowed to fall into ruin dependent upon the dynasty ruling China. The Qin, Han and Ming Dynasties felt the Wall was a critical part of the national defense while the Yuan abandoned the Wall as useless (being the very "Barbarians" that the wall was designed to contain this was a logical move). Most of the Wall that is visible today are sections the Ming built-rebuilt in the 16th-17 centuries. The Great Wall was finally abandoned by the Qing Dynasty in the mid-1600's for much the same reasons as did the Yuan. In the end the Wall was only as good a defense as the soldiers that manned its bastions. When the quality of the army declined the defensive value of the Wall declined as well, soon the Wall was breached and the dynasty it defended died.