This game has a lot of potential, just needs some polishing. I like the way you handle upgrades, much more interesting than most TD games. Obviously the graphics can stand to be improved.
Seeing the stats when you click the tower, bigger maps, more creeps, more towers, etc, would all be nice. I would like to see how much the +Damage actually adds.
Basically the more you can expand on the concept, the better. Looking forward to what comes next.