Need to be able to look around the level. Needs a clearer indication of when it's your shot. Rather than leaving the arrow where it was for the last shot, how about a line or some such indicating it? As it is, half the skill in beating it is not moving the mouse between shots.
Nice concept, bit buggy and you need to look at balance/difficulty. Some warning for the two tank mission would be nice - all you can do is loose unless you bought two tanks to start with.