Mexico City, formerly known as Tenochtitlan, the capital of the Aztecs, was built on an island in Lake Texcoco and one of the world's largest cities by the time it was discovered by Hernán Cortés in 1519. Eyewitness Bernal Díaz del Castillo described the awe of the Spanish upon their discovery:
And when we saw all those towns and villages built in the water, and other great towns on dry land, and that straight and level causeway leading to Mexico, we were astounded. These great towns... and buildings rising from the water, all made of stone, seemed like an enchanted vision... Indeed some of our soldiers asked whether it was not all a dream... It was all so wonderful that I do not know how to describe this first glimpse of things never heard of, seen, or dreamed of before.
According to legend, the Aztec ruler of the time, Moctezuma II, believed Cortés to be the prophesized incarnatation of Quetzalcoatl, causing the Aztecs to be easily defeated. However, in reality it took a months-long siege before Tenochtitlan was finally conquered in 1521. The city was destroyed and rebuilt as Mexico City - now again one of the largest cities in the world...
In rebuilding the city, the Spanish kept the street grid created by the Aztecs, and they established a church at the site of the former Great Pyramid or Templo Major. Though some remains of this structure have been excavated, they are not very spectacular and have mainly symbolical value. The location itself is highly significant, since it was here that the wandering Aztecs found the destined site for their capital when they saw an eagle perched on a cactus with a snake in its beak. This vision now adorns the Mexican flag (seen below at the daily ceremony on the Zócalo).
The Templo Major, dedicated to Huitzilopochtli (the god of war) and Tlaloc (the god of rain and fertility), was also one of the places where the Aztecs practiced their infamous human sacrifices. After the final enlargement of the temple in 1487, some 80,000 people were sacrificed over 4 days, according to Aztec records (modern estimates are somewhat lower). In order to acquire enough victims for these ceremonies, the Aztecs waged ritual wars known as the Flower Wars.
Update: As a counterbalance to the Aztec sacrificial practices, a good article introducing their poetry is 'The Aztec Way of Poetry'. In Aztec thought, the Nahuatl phrase "in xochitl in cuicatl" ("the flower, the song") summed up poetry, or art in general. As the article recounts, poet-king Tecayehuatzin once summoned poets and wise men to discuss the meaning of poetry. He said: "Flowers and songs, or, it may be, art and poetry; is this perhaps the only truth here on earth? Or perhaps flower and song are the only means of expressing true words."
Here's a sample from a poem (from this Nahuatl Poetry page), showing also the close connection between flowers and war:
Like a wind lilly the shield turns,
like smoke, the dust lifted,
the whistle with the hands repercutes,
in Tenochtitlan México;
where the place of the Tigers is,
the ones who have the charge of war
whistle with the hands for the battle.
Ah, the flowers of the Smoking Shield,
it is not true, it is not true,
they will never cease, they will never finish!
Though I may cry, though I may worry,
as much as my heart does not want it,
will I not have to go to the Mystery Region?