Walls, as a defensive measure was a rarity in Mesoamerican cities, to my knowledge the aztecs only faced a siege on 2 ocassions, against the Zapotecs and against Huecotzingo, in both cases the "walls" were more akin to palizades than to walls and had been quickly built specifically for that conflict.

In reality, like in Europe during the Pax Romana, there was little need to build walls or defenses of the type, most conflicts were resolved on the battlefield, and war was a very ritualistic affair, defeat conditions being very specific.

The Aztecs were in the tradition of building "fortress" towns and states though, now these werent forts or castles in the european tradition, though there were outposts like those, in which case, defenders would usually be composed of infantry and ranged Atlatl users positioned along garrisons and strong points, examples of this are Oztoma and Quauhtochco but the main defenses rather than buildings were border states used to create a buffer zone between the aztecs and their enemies, giving them time to gather a force in response, or launch an attack from... think the Warsaw Pact or the Neutral Zone from Star Trek, rather than an outright wall on a frontier.