Currently BuildingAI treats gaia different than other players, in particular ignoring healers and enemy towers for example. This is particularly annoying on Danubius where your units can chase healers all over the map; it would be nice if a tower could take them out.
We should attack any damage receiver except non-dangerous animals, since it's a bit silly to attack your starting chickens. There's a bit of a loophole with defensive animals like gaia elephants, which aren't dangerous (violent or aggressive) but can attack back. Since we can't manually control BuildingAI (yet, anyway), the alternative is to make them also attack those by default, but it seems bad to pick a fight with an elephant who will then destroy your tower.
Hmm, actually testing now, and elephants don't attack back. Seems like a bug? And I thought you used to be able to trick an elephant into attacking an enemy's civic center? Maybe later I'll explore what's going on with this stuff...