nice shell!
Ok I did enjoy the game it did help me in my menu analysis and design class for sure, but the problems with it were noticable.
You really need to change the art style once you upgrade something, it gives you some small sense of accomplishment, almost like I have to upgrade to see what it will look like next.
Try putting the enemies harder, or almost make it so that you have a choice of going for the hard baddies or making it a slaughter fest and getting everything on the map.
The controls were very user friendly and I wouldnt change those at all, good job and hope to see some more!