Pretty decent, needs some more work.
Good tower defense game. Needs some more towers, though. Stuck with 6 towers is a little boring.
I'd also suggest changing the way you earn your money. A weak enemy giving the same amount of money as a stronger enemy seems a little unfair to me. Another suggestion, if I may, involves the end-wave bonus. While I wouldn't think a steadily-increasing bonus is bad, I think it's better suited for a tactical game like this to involve an interest-based reward system. Maybe half-and-half, where part of the end-wave bonus is set and part is interest-based. Just my thought.
A selection of music would also go a long way, too. Listening to a minute-long loop over and over again gets old.
My impression of it now is that it's a promising tower defense that has the chance to become something great. I hope you find better ideas than what I've proposed and further improve you game. Good luck and I look forward to playing an updated version.