The altepetl, literally 'water-mountain', was the fundamental ethnic city-state of central Mexico: a sovereign people with a dynastic ruler (tlatoani), a patron deity and a defined territory centred on a town. Each altepetl was built from rotating constituent districts (calpolli) that shared the burdens of labour, tribute, military service and temple duty in turn. Tenochtitlan was itself an altepetl, and the empire was a hegemony of many altepetl bound by tribute rather than a unitary state. The model organised both the internal layout of towns and the political map of the valley.
It is the cellular political-territorial unit that structured Mesoamerican urban society and survived into the colonial order as the basis of towns and parishes. The altepetl shows a distinctive, modular conception of the city as polity.