Nice submission! I love tower defense games, and I had a lot of fun playing this :)

The progression of the game is a bit too fast: the second level felt much harder in comparison to the first one, and I would have preferred a slower pace.  However, perfect balance is hard to achieve in a handful of days. So, don't worry, the gameplay remains interesting and solid ;)

I also experienced the same bug described in other comments; I hope you can fix it in the next release! Anyway, congratulations for your entry!