if you are going to do this, it should have a food cost and a limit of 1 per human, or be really expensive. You can do a lot with a "hero" tower, and its a novel idea (I think... but I saw something like it in "crazy's vampirism"), but there are a lot of things that you COULD put into the game that would just ruin it. I'm not saying that a "hero" tower is one of those things, but you need to consider it before you rush off to add every new idea.