I'm not sure Tenochtitlan was technically a city.

The defining characteristic of a city is that it can't feed itself. (Hence the "urban" vs "rural" dichotomy.)

Correct me if I'm wrong, but Tenochtitlan was a closed system and used to have farms on the lake and the islands.

Neither of the other city states in the Aztec Triple Alliance had walls either and they weren’t built on islands.[1] They seem to have had some walled precincts to separate sacred spaces from common areas but now broader system of defensive walls. Much like Ancient Rome they had large and well organized armies.

[1] https://www.public.asu.edu/~mesmith9/1-CompleteSet/MES-SAA-0...