Ancient civilizations built underground cities. Why did they choose subterranean sanctuaries over surface living? Ancient civilizations built vast subterranean cities primarily for protection against invasions and harsh climates. These complexes were not mere shelters but sophisticated urban systems reflecting advanced engineering. Construction spanned millennia, from the 3rd century BCE with the town of Naours in France, which began as a Roman quarry, to the 7th and 8th centuries CE with the mu