I don't think they are cannons, due to the projections on the bottom. I think they are in a class often referred to as "wall guns", the loaded gun was fired from the top of parapet walls (or handy log,boulder,etc) with the projection on the gun on the enemy's side to take the recoil. They were like primitive "punt guns" in use, but instead of ducks and geese, they were pointed at Aztecs.
Mike
If the ducks, geese and Aztecs compare notes in the hereafter, I question if significant differences will be noted.
Best,
Ted
