A very good attempt. Graphics were robust for their use, the learning curve was satisfactory and the music was decent. In my opinion, with a few additions this could become a really great game. Here are a few things that have room for improvement:
1. The difficulty, at least on normal, might need to be lowerer just a bit. The building strategy felt unforgiving - one small mistake and you definately loose a life, of which you only have 5.
2. The tower statistics are only presented for the upgrades. It might be useful to provide statistics of a tower on click or rollover, for informative reasons.
3. Some enemy (like the running-dog-thing) seem more appropriate for later levels, not too early on (were the mazing is not yet developed).
4. Though I did not mind the single-stage availability, you could at least add some variation in the texturing or enemy (no real stats differences, just different sprites) and present it as a different level; the variety might keep the player interested for just a little bit more.
Cheers, we expect to see more!