Mexico City, Mexico, satellite image. Mexico City, located in southern central Mexico, is the country's capital and largest city. The city was built in the 16th century by the Spanish, on the ruins of the Aztec city of Tenochtitlan. At centre is Zocalo Square. It is the largest square in Mexico City and dates back to the Aztec period. At the top of the square is the Metropolitan cathedral and at left is the Palace of Government. Mexico city has a population of over 18 million people, making it one of the most populated cities in the world, and severely overcrowded. Taken by the Ikonos satellite on 7 August 2003.