The heart of Mexico City: the Zócalo (Plaza de la Constitución). Sitting at 2,240 m elevation, it's one of the largest plazas in the world. Surrounded on all sides by Baroque historical buildings, you can feel the layered history of Mexico simply by standing here.
The Metropolitan Cathedral
Facing the Zócalo, the Metropolitan Cathedral (Catedral Metropolitana) took about 240 years to build (from 1573) and is the largest Catholic cathedral in Latin America. The exterior layers Baroque, Renaissance, and Neoclassical styles — a product of that long construction.
The whole structure leans because of subsidence. Walking inside, you can feel the floor's faint slope. That itself tells the story of this city's geological layers — Tenochtitlán, the Aztec capital, once stood here, and the Spanish built their colonial city on top of it.
Templo Mayor — the Aztec Central Temple
Right next to the cathedral, Templo Mayor was the central temple of Tenochtitlán, the capital of the 14th-century Aztec empire. After the Spanish conquest, the temple was destroyed and its stones reused in the cathedral's construction. Its existence lay buried until 1978, when it was accidentally rediscovered during underground works.
The on-site Templo Mayor Museum displays a vast number of artifacts. The sculptures concentrating the Aztec worldview are remarkable. The cross-section model of the temple shows clearly how the structure was built and rebuilt in layers.
Palacio Postal — A Hidden Gem of Architecture
About a 5-minute walk from Templo Mayor, the Palacio Postal (1907) is an Art Nouveau post office designed by the Italian architect Adamo Boari. It still functions as a working post office, but inside it looks like a European palace. Visiting is generally free and the interior is open (range may vary by tour).
It's said that this building, together with the neighboring Palacio de Bellas Artes (Palace of Fine Arts), inspired some of the visual design of Pixar's "Coco." Anyone who's seen the film may notice familiar details.
Walking around the Zócalo is most interesting when you keep "layers" in mind. A Spanish cathedral built on Aztec stones, an Art Nouveau building from the 1900s right next door. Mexico City is a cross-section of history all by itself.