Hm. I think trying to give shooting a second purpose is not really working here. You shoot the enemies and if you were unlucky you have to shoot once again after that, to "up" the walls. If you get even more unlucky, you have to reload to "up" the walls, but that triggers enemy spawning once again. When playing, i didnt really get into the flow, because i had the feeling the game was holding me back every now and then.
What i like is the hook mechanic and generally, connecting shots to world events seems like an idea that could work well. Maybe if you wouldnt have to reload, but the enemys spawn every 10 Shots and you have only lets say 50 shots for one level. So you really have to manage your ammunition but dont get "hold back" by reloading or enemies spawning without any alternative.