We actually know very little about the founding of the great city of Central Mexico. Of the buildings that remain, the oldest seem to be from about 200 BC (BCE). But it seems that it was a city of great power between 150 BC and 750 AD. At this time it became the 6th largest city in the world (after Constantinople, Changan, Loyang, Ctesiphon, and Alexandria). It’s believed that the population was around 125,000 and up to 200,000.
Researches have speculated that several small settlements may have joined around the lava tube cave, over which the sun pyramid (temple) was eventually built. Caves often held important ritual significance to the peoples in the area. This one may have become an important shrine because it had four chambers – a symbolic number.
What we do know is that the city was brilliantly constructed. Large buildings and small, the wide central avenue, and careful organization are a part of what makes the ancient city stand out. The city has been carefully studied, and researchers believe they have found at least 6 distinct social “classes” in the buildings of Teotihuacan (according to RF Millon).