I find the implementation of the theme too weak. But I have enjoyed the graphics very much. The style is consistent and the effects are really satistifing. So the scene setup is really nice.
I could imagine some dark/cellar-like sounds and upgrades would be nice. May the enemies could spawn in waves and the player can decide between random upgrade-options.
Also some variations of the enemies would be nice. Like stronger ones which require more damage (may indicated by different color. E.g.: they start with yellow, first bullet hit turns them green, next bullet hit kills them).
In order to increase the difficulty you also could manipulate the arena. Like letting obstacles or traps appear.