The Great Wall of China was built across China.
The Great Wall is a connected system of fortifications constructed, repaired, and expanded over many centuries. Some early walls date to the period of the Warring States, before China was unified. The first emperor, Qin Shi Huang, linked sections after unification in the 3rd century BCE, while the best-preserved and most famous sections were built or strengthened during the Ming dynasty from the 14th to 17th centuries.
The wall’s purposes included border defense, troop movement, surveillance, and control of trade and travel. It is not a single continuous wall made at one time. Its remains include walls, trenches, passes, watchtowers, and natural barriers.
The popular claim that the Great Wall is visible from the Moon with the naked eye is false. Its materials and narrow width make it difficult to distinguish from orbit without optical aid, despite its immense total length.