China contains the Great Wall, a vast system of fortifications built across northern parts of the country. It is not one continuous wall constructed at a single time, but a network of walls, trenches, passes, and related defenses.
Several Chinese states built early defensive walls before the Qin dynasty unified sections in the third century BCE. Later dynasties expanded or rebuilt the system, with many of the best-preserved stone and brick sections dating from the Ming dynasty, which ruled from 1368 to 1644.
The wall helped control movement, protect agricultural regions, and regulate trade along routes near the frontier. Its effectiveness varied, and it did not make China permanently invulnerable to invasion. The Manchu-led Qing dynasty eventually entered China in the seventeenth century.
A widely cited survey gives the entire Great Wall system a length of more than 21,000 kilometres. The claim that it can be seen from the Moon unaided is a myth; its width and colour make that impossible from such a distance.