Cool game. I especially like the music. A few inputs:
I had still fun playing it. Congratulations to a solid entry!
After reading the other comments I played again and difficulty is not too high :). Forget about it.
It would be cool if you could see the level of your towers without the need to click on them, e.g. by showing the level always above them or so. Additionally it quite often happened to me that I accidentially placed a tower and the inidication if you can update a tower or not based on your money (white or red font) does not update properly.
Don't take me wrong, these are just a few issues which could be improved. I still liked the game overall.
