Very nice, few things that could be better though
Firs of all, very nice game and very enjoyable. Few things that could be better though:
1) The "unlock available" button doesn't catch your eye, for me at least, and when I figured I missed the opportunity to unlock something and went to unlock it, I found out I can't. I think that either the fact you can unlock something isn't shown enough or the unlocks should be saved for later use if you don't use them.
2) I was frustrated to start a level from the whole beginning when I died, especially the first General level (I haven't finished the game yet, and to tell the truth I don't think I'll do it now). It could be a lot more helpful if there was some kind of checkpoint, even just one at the beginning of the boss. I did the same level 3 times already and died each time when already reached the boss.
All in all, very nice game and I liked the graphics and variety of enemies you have to fight. Very good job on that.