Using your health bar for everything - to shoot and buy stuff is an excellent idea! The implementation has issues though. The damage upgrade doesn't have any purpose since all enemies died from 1 hit. Also, the final goal of 2000 souls is way too long unless you have more varying mechanics and monsters. I hope you make a better version of this, because the idea itself is great.
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Yeah, technically the enemies do get harder over time and require more hits to take down (enemy difficulty increases every 5 minutes), so there is a reason for the upgrade, but let's be real, it's a poor implementation. But yeah, the idea does have merit. It just needs more polish, balancing, and variety, among other things.
Thanks for playing my game!