China built the Great Wall, the world's longest defensive structure.
The Great Wall is a network of walls, fortifications, trenches, and natural barriers constructed and rebuilt over many centuries. Earlier walls existed before the Qin dynasty, but Qin Shi Huang ordered the connection of several northern defenses after unifying China in the third century BCE.
Many of the best-preserved sections date from the Ming dynasty, which ruled from 1368 to 1644. These sections often use brick and stone and include watchtowers, passes, and signaling systems. The wall helped control movement and protect agricultural regions, though it never formed an impenetrable barrier.
The entire network, including all branches, is measured at more than 21,000 kilometres. A popular myth says it can be seen from the Moon with the naked eye; it cannot. The wall's route crosses mountains, deserts, grasslands, and plateaus.