First of all, I found the introduction very cool ! But I think that this is the only place in the game where you feel like the bad guy.
For the gameplay, I agree with the posts made earlier : a background music is important, and the limitation isn't very present, and it was also kind of boring after the wave 15, maybe you should have added another map and make the wave half as long ! It would have been refreshing I think.
At the beggining of the game, I had some fun tho ! Overall I'm mixed