Teotihuacan's residential compounds are one of the most singular urban innovations of the ancient world. They were compact blocks of roughly 60 meters per side, walled, with their own shrine, workshops, and storage areas—a unit that integrated housing, production, and cult in a single enclosure. What sets them apart from other forms of collective housing is that each had its own identity: some were dedicated to obsidian work, others to pottery, others to lapidary work. Isotopic analysis of residents' bones confirms that many complexes were multiethnic, with people from different origins living together. That organization—diverse, specialized, and self-sufficient—was possibly the key to the city's stability over more than 500 years.