Suffocate your enemies with this strategy: http://oi61.tinypic.com/28utjxt.jpg
It's actually a great way to get gold if you have the landlord upgrade, and it gives you even more gold if you have the assassin upgrade.
I dominated the later levels by just filling up the whole map with towers.
Nice game. Felt like another boring half-assed clone at first, but it turned out to be inspired and original. Good enough to keep me playing to the end. I got all the awards, but I'm not going to bother getting all the stars because some of those stars are just obnoxious.