Really great game and idea you've got going on here. Some suggestions that may be more than obvious / already are in the works for a sequel should you make one (which it sounds like you do which makes me and everyone else really happy):
More enemy types, more backgrounds/levels (maybe the top of the first tower/segment just leads to the next one and they get increasingly more difficult/taller), more normal attack abilities than just a single gun, different special abilities besides just the dash/double jump, and maybe some interesting critical health near-death abilities, like traversing some smaller walls/obstacle when youre on the last heart. Stuff like that, but I'm sure you've had most of these ideas since they're pretty standard when it comes to sequels.
Still, great stuff you have here and can't wait to see what else you do.
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