At its peak, Tenochtitlan was one of the largest cities in the world, even larger than London at the time!

Tenochtitlan was the capital city of the Aztec civilization. They saw a vision in the Lake Texcoco that is now immortalized in the Mexican flag by a hawk eating a snake. It was there that they built their city. 

Their culture thrived and they began to dominate other tribes. The city grew and became the largest and most powerful in Mesoamerica. Commercial routes were built to the city from places as far away as the Gulf of Mexico and the Pacific Ocean.

Some even think it went all the way to the Incan Empire. After a flood came on Lake Texcoco, the city was rebuilt. The ruler, Ahuitzotl, had the city rebuilt in a way that made it one of the grandest ever in Mesoamerica. 

The most common estimates say that over 200,000 people lived in Tenochtitlan. Only Paris, Venice, and Constantinople were thought to be larger than Tenochtitlan in 1519 when Hernan Cortes arrived to the city. 

(Source)