London was like a dot on the map compared to where the Aztecs lived. The Aztecs had an empire. To compare like with like, the Aztec empire should be compared with the Roman Empire, rather than the city of Rome. The Roman Empire was 30 times bigger than the Aztec Empire.
When Cortes appears at the Aztecs empire, they believe that he is the god, Quetzalcoatl but he is truly an enemy that is out to destroy the Aztec Empire. Anyway, the Aztecs welcome the Spanish with food, and with a place to stay that lines the entire city giving an easy way to defeat the Aztecs.
Empire is partly correct as it was an Empire but that isn't a relevant answer. As far as i know and i like studying about the aztecs, no actual group of people founded the aztecs it was a sort of culture that just slowly came to be.