I've also made a tower defense game and i know how hard it is to develop in 72 hours. Balancing in particular is a big issue.
However i like the concept of your game, where you have to let enemies through, in order to survive, but just enough that you don't get over 200 lives. This concept opens a whole new strategys to consider.
Unfortunately the UI is a bit small, at first i didn't even see where i can see my money.
All in all this game is very fun, with a simple design. I respect that you managed to make a tower defense game. Great job!