Pretty cool game, i enjoyed playing it a lot. The mechanic with 3 upgrade slots for each tower is really cool, it makes the game really interesting and forces you to figure out which upgrade is best where. I also like the fact that you decided to go for fewer waves and then introduce new levels. It keeps you from fortifying your position so much that the game becomes boring. Well done.
I beat most of the levels at the first or second try, just the last one was more challenging, which was a welcome change.