Three cultures converge here: Aztec, Spanish, and contemporary Mexican. Surrounded by modern office and apartment buildings are large remains of the Aztec city of Tlatelolco, site of the last battle of the conquest of Mexico. To one side is the Church of Santiago. During the Aztec Empire, Tlatelolco was on the edge of Lake Texcoco, linked to the Aztec capital by a causeway. Bernal Díaz de Castillo, in his True Story of the Conquest of New Spain, described the roar from the dazzling market there, and the incredible scene after the last battle of the conquest in Tlatelolco on August 13, 1521 -- the dead bodies were piled so deep that walking there was impossible. That night determined the fate of the country and completed the Spanish takeover of Mexico. It was also here, in October 1968, that government troops fired on protesters who filled the square, killing hundreds.

View the pyramidal remains from raised walkways over the site. The church, off to one side, was built in the 16th century entirely of volcanic stone. The interior has been tastefully restored, preserving little patches of fresco in stark-white plaster walls, with a few deep-blue stained-glass windows and an unadorned stone altar. Sunday is a good day to combine a visit here with one to the Lagunilla street market, which is within walking distance, south across Reforma.