Cortés had marched inland from the coast with a few hundred Spaniards and thousands of Tlaxcalan allies. Moctezuma met him on the causeway into the city, which stood on an island in Lake Texcoco and was larger than any city in Spain.
The Spaniards were housed as guests, but within days Cortés took Moctezuma prisoner. Fighting broke out the next year, Moctezuma died, and after a long siege the city fell in August 1521. Mexico City was built on its ruins.
Source: Wikipedia