The Aztec capital, built on islands in Lake Texcoco (1325 AD), which once was the center of the Valley of Mexico; few remains survive underneath present-day Mexico City. The Aztecs built artificial islands and constructed houses, other buildings, and chinampas them and then connected to the mainland by three giant causeways. The population may have been as high as 400,000 people over five square miles. Under the ruler Itzcóatl (1428-1440), Tenochtitlán formed alliances with the neighboring states of Texcoco and Tlacopan and became the dominant power in central Mexico. By commerce and conquest, Tenochtitlán came to rule an empire of 400-500 small states --by 1519 some 5-6,000,000 people over 80,000 square miles. Accounts describe 25 pyramid-temples with nine priests' quarters, seven tzompantli (sacrificial racks), two ball courts, and a huge plaza consisting of the Great Temple with the temples of Tlaloc and Huitzilopochtli. The city was taken by Hernando Cortes and the Spaniards in 1519 and by 1522 it was virtually destroyed. The Spaniards built their own city on the site. Some archaeological remains were discovered during the building of a subway in Mexico City.