Good but....
The game is fun but WAY to easy no matter what pretty much, some things need to be able to be changed... such as the upgrade cost and it's % of stat increase (yes I went to game options to change it but it kept going back to 20% which is way to much for some maps). The limits of the stats are also kinda lame so as you cannot upgrade past 30damage/speed and you can only set base stat so low.
A cool concept you could toss in is a drawing feature, instead of only being able to pick your premade drawings for towers/enimies why not make it so the player can draw there own things with a space limit with titles too even, that'd be bad ass if possible.
Make some of these changes and I think you'll have one hell of a game and then some (because this is already a damn good game, just with some flaws). I love the concept and everything else though.
Keep up the good ideas and work.