 Zone B covers the areas all other constructions to the end of the 19th century that are considered indispensable to the preservation of the area's architectural and cultural heritage.This is where the Spaniards began to build what is now modern Mexico City in the 16th century on the ruins of the conquered Tenochtitlan, capital of the Aztec Empire.As the centre of the ancient Aztec Empire and the seat of power for the Spanish colony of New Spain, the Centro Historico contains most of the city's historic sites from both eras as well as a large number of museums.