The towns linked by causeways to Tenochtitlan

You’re welcome, and good luck with your book!

Great question! We can’t find any term in the literature to define something in between a) an altepetl - usually translated as ‘city-state’ and b) calpolli - usually translated as ‘ward’ or ‘neighbourhood’. The larger towns listed here, such as Atzcapotzalco and Tenayuca would certainly have been altepetl.
We think the best explanation of these units comes in the ‘new’ (2021) book by Berdan & Smith ‘Everyday Life in the Aztec World’:-
a) ‘To quality as a city-state, a polity must have “a legitimate ruling dynasty, a sense (if not the actuality) of political autonomy, control over local lands and labour, a well-established founding legend, often with mythological underpinnings, and a patron deity complete with temple”. Some also contain a dominant ethnic group, feature an economic specialisation (such as feather-working or ceramics), and “may have enjoyed renown as extraordinary market or pilgrimage destinations”... Each city-state is headed by a tlatoani, a ruler who commands the allegiance and loyalty of the altepetl’s residents who go to war on its behalf. While city-states vary considerably in size and influence, “a typical altepetl in the Basin of Mexico had a population of 10,000-15,000 and covered an area of 70-100 square km”.
b) ‘A calpolli was a group of households whose members lived near one another and typically shared some key economic characteristics and social relations. In cities calpolli served as spatial divisions, or neighbourhoods. Calpolli often featured specialised craft occupations and their members shared a common patron god. A house cluster typically had some twenty to twenty-five households, situated near one another. A calpolli would contain several house clusters, and contain fifty to a hundred households. The (large) calpolli usually had a common temple, perhaps dedicated to a patron god. The calpolli also had its own school. In rural areas, a calpolli was usually an entire village or town, with its temple and school, as in the urban setting.’
SO, it looks like a small town of - say - some 4,000 inhabitants could in theory either be a large calpolli or a small altepetl...!

This would require a lot of extra research time which we don’t have at the moment, but we’ll keep this on the ‘to do’ list...!