When the Aztecs founded Tenochtitlán in 1325, they built it on a large island on Lake Texcoco. Its eventual 200,000-plus inhabitants relied on canals, levees, dikes, floating gardens, aqueducts and bridges for defence, transportation, flood control, drinking water and food. After the Spaniards conquered the city in 1521, they drained the lake and built Mexico City over it.