The game is really fun to play, love the implementation of tower defence concept here. Both the graphics and the sounds were great. I've had some minor issuses though - using Shift to perform actions is not the best idea for Windows because it triggers some annoying system popups. Also I couldn't tell when I could build new defences or when was the time to maintain existing ones or even when I was about to 'die' - the game could give a player better feedback on that.
Overall, it was a really fun experience, good job.