I played this yesterday evening - long enough to realise I should really head to bed. I had a lot of fun - tower defense games always do me like that, even the small ones.
I liked the mechanics of having foundations/towers separately. That could be expanded upon very well, with synergies and the likes!
I did end up wondering - is there a hp mechanic for the towers? There's several things that seem to imply it, like the pentagon foundation, but at least as far as I got, none of the creeps ever seemed to attack my towers or anything like that.
Good work on this, was a lot of fun!