I like top down shooters, and I thought someone might make one with the sprites in the pack, so that's cool to see.
This is pretty rough around the edges, but you've laid down some solid core mechanics (shooting, health). I think if you focus on polishing those by doing some of the following, you could create an engaging game:
- Lower enemy speed to give the player a decent chance to shoot
- Create different situations for the player to react to (i.e. where they trigger multiple enemies at a time, instead of just one after another)
- Different enemy types with different behaviour
- Different weapons/powerups/pickups for the player to use
Once you have those down, you can just focus on polishing and juicing your game, with some extra animations and visual effects.
Hopefully that helps. Would be cool to see you do any work on this in future. Keep it up.