The Plaza de las Tres Culturas is built around the ruins of Tlatelolco — once a separate Aztec city across the lake from Tenochtitlan, with the largest market in Mesoamerica. The Aztec pyramid base survives at one end of the square; a colonial-era Franciscan church of Santiago stands directly over what remained of the temple after the Spanish dismantled the upper levels in 1521. A 1960s housing complex frames the whole.
The square is also the site of the Tlatelolco massacre of 1968. A memorial along the eastern edge of the plaza names the victims who could be identified. The square is reached by taking the metro to Tlatelolco station. The site is free; a small museum on one corner sets out the history.