Really nice look and feel to this game. Art style look great overall, and music fits nicely into it.
You achieved a lot during the jam time frame, so my improvement notes are more for moving forwards.
Obviously need power ups :), think that's probably been mentioned before.
Bit of balance on how many shots things take to destroy (combined with any power up mechanics above).
Top and bottom sections can be used to hide in a bit at times, especially if you can get ahead of the enemy waves. Consider if this is a keeper, or not. Combined with a thought about the gun, and wether there should be a restriction on how far it rotates backwards (maybe leave a blind spot behind it can't do?).
Great work on this. You've done a really good jam game entry, one of my favs. But also built a strong base to build from to take this game further.