A few suggestions.
1) Move the jump button to the up key. Jumping with Z feels a little un-intuitive. With shooters, intuitive controls are key.
2) Everything chrispy said.
3) Unless it's a key part of the game's design, maybe add mouse aiming? It would free you to add bosses that don't have to be in front of you the entire time.
I like the graphical ideas I've seen so far. Run with it. Also, and this is just cosmetic- maybe update the enemies so their sprites respond to your position?
hellhawk-
run to the edge and jump off. rinse and repeat until you can start jumping off the heads of the falling shadows.
also, magnificent game. 5/5. I'd love to play another game like this *hint hint*
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