Nice little game, there are a few "bugs", although they could be intentional (mostly physics stuff). Love the idea of using health to upgrade, as most people in the comment section. I'm not sure I found the easter egg though sadly :( I also thought the shooting ability was broken until I bought it and I finally could shoot! Good job!
Note: This is my personal opinion, but I feel like I would've preferred a faster attack. The sword was moving kinda slow and the animation felt a bit weird to me. I think it would've look really good if it was way faster. Maybe do much less damage, but be able to spam the attack a bit. If it was a design decision then it's fair, I'm just giving my opinion :)