Tornado does have a 1-season cooldown.  However, it is cast per-champion, so you can cast it once per turn per champion that has it.  Having it happen that much, though... you might be running into a bug of some kind (I guess that's why you have [Bug] in your subject line).

After defeating every monster inside the Imperium from the inside out, there were no locations available to build.

I don't think it was possible that Altar had seven different heroes able to cast tornado.  And I am not talking about once, I am talking about EVERY round.  Granted it was very late game and it's entirely possible he had built up enough mana to cast it up to spam it constantly making it impossible for anyone to attack him.