Idea is there, setting feels good, but there are some things lacking.
Enemies should do something when killed, except just disappearing - calling Singleton controller to play "enemy-dead" sound is quick and easy to add a lot to how good damaging enemies feels. Add simple disappear animation and you have all you need for that. Also I believe bullet should be destroyed after it collides with enemy. You can add piercing ammo as a unlock/power up.
You can only move left, right, up, down. Not diagonally. It's an odd choice for this kind of game.
With that said, good job on finishing the game, as this is the most difficult part. I'm sure you will have this figured out for your next game. Keep it up!