Okay. if you don't add secondary weapon, an upgrade or shop system after the levels would be good for deeper gameplay.
Here's some feedback after playing further:
1. you have to introduce new enemies faster. level 1~3 contains only 1 kinds of enemy which is quite boring. If I were you, I'd add new enemies after every waves and start to increase the difficulty once all enemies are introduced.
2. Exploding enemies are quite boring to fight. I have to shoot it once and wait until it explodes. Instead of sitting duck a creeper-type enemy that chases me and then self-destruct would be more interesting. of course killing it shouldn't cause damaging exlosions.
3. I can barely find the heath in the levels, it's very frustrating in later levels. you should give a small amount of guaranted health bonus in all levels.
4. Enemies should be smarter as the game progress, like anticipating the player's location based on its movement speed and distance when firing bullets, and actively avoiding the player's bullets. 6DOF games need smart enemies to fully utilize 6DOF controls.
5. wouldn't it make more sence if turrets are mounted on walls instead of floating and not moving at all ?