Every town in México has it’s zócalo. Usually there is a cathedral on one side, a government building or two on another, and a plaza in the middle. México City is a little different as it has an archaeology site as well! The original city, Tenochtitlan, was razed by the Spanish in 1521 so they could build the cathedral and other buildings. They didn’t do a great job, and much of the antiquities lie just below the foundations of the colonial buildings in México City’s Centro.
The Templo Mayor is located just next to the Cathedral. As of this post, admission was 75 pesos. INAPAM cardholders enjoy free admission. One of the most accessible and interesting sites in the city.