First I would like to say, I am no a game writer; I am a game player. I acknowledge the difficult in constructing a game like this. I've noticed that often criticisms on this site get voted "useless". I choose to share this following criticism despite this, because I have just wasted my time playing this, and want fellow game players to be warned.
In my opinion, this is just another tower defense game with nothing new to offer. And on top of that, the upgrades are weak and ineffective! For example, a sniper tower costs $250 to build; the reload time is 50, and paying $500 to upgrade this towers reload, it becomes 48. 48?!! For twice the cost of a new tower?!? A new tower would cut the total reload time in half, between the two of them. Also, the expensive towers costing $1250 and up have similar, if not lower, general damage levels than those that cost less than $1000 (I acknowledge the advantages of the freeze and shock towers as being an exception). I could handle the first level, up until about wave 26, before I was sick of waiting for something interesting to happen while I filled the screen up with machine gun turrets and sniper turrets; and I was very patient! I don't mean to just rag on this game; I think the coding is decent, and the graphics are decent, however the numbers involved in upgrades are ineffective and make upgrading a disappointing practice of trial-and-error. If you remake this, adjust the numbers, and I would suggest displaying the prospective upgrade adjustments before the player has to pay for the upgrade. Also... think outside the box a little. 3 stars for the effort involved.
And one last thing... pretending to be "loading new level" to show an ad is really weak!