Holy crap, WTF were you thinking?
I love TD games. I've found a few that rocked my sox, and a few -like this- that made me scratch my head and want to throw poop at my monitor.
Your critters are too strong, your towers too weak. Your starting cash gives you very little to work with in terms of offensive capabilities. Hell, dude. I CHEATED on this game, and was barely able to survive the final round. I had towers on every piece of real estate possible, and I still got damaged. Seriously consider re-tooling your mechanics, and adding possible tower upgrades to make it more playable. I'm not saying easier, just more enjoyable. Nobody likes to be totally creamed with little chance of success.
Harsh rating, I know, but like Oracun, I have high standards of playability, and this doesn't get there.