When Cortes and his Spanish missionaries converted the Aztec in the 16th century, they tore down their temples and used much of the stone to construct a church on the site. Nearly all of the stone from the nearby Templo Mayor was built into the cathedral. The original Spanish church was torn down in 1628 while the present Metropolitan Cathedral was under construction. The towering cathedral that stands today, begun in 1567 and finished in 1788, is a blend of baroque, neoclassic, and Mexican churrigueresque architecture.
Over the centuries, the cathedral and the Sagrario (chapel) next to it have sunk into the soft lake bottom beneath. The base of the facade is far from level and straight, and when one considers theimmense weight of the towers —127,000 tons — it's no surprise. However, much to the credit of Mexico City and its preservation efforts, the Catedral Metropolitana came off the World Monuments Fund's list of 100 Most Endangered Sites in 2000, as a result of an extensive reconstruction of the building's foundation.
The exterior of the basilica is a medley of architectural styles. Inside, four identical domes are supported by rows of columns. There are 5 naves and 14 chapels, mostly designed in the ornate churrigueresque style (named for Spanish architect José Churriguera, who died in 1725). Like most Mexican churches, the cathedral is all but overwhelmed by innumerable paintings, altarpieces, and statues in full graphic color.
The Metropolitan Cathedral contains many prized works of art from the colonial era, in a variety of artistic styles. Jerónimo de Balbas built and carved the Altar de los Reyes (Altar of Kings) and the Altar del Perdón (Altar of Pardon) in 1737. Among the cathedral's other outstanding features are: the tomb of Agustín Iturbide (1838); a painting attributed to the Spanish artist Bartolomé Esteban Murillo; and the fact that the stone holy-water fonts ring like metal when tapped with a coin.
Like many huge churches, the cathedral has catacombs underneath. In front of the cathedral you can find various crystals, gemstones, and herbs for sale, which are believed to provide special qualities of protection and cure from various afflictions.
The much older-looking church next to the cathedral is the chapel known as the Sagrario, another tour de force of Mexican baroque architecture built in the mid-1700s." - in: http://www.sacred-destinations.com/mexico/mexico-city-cathedral.htm