I love it. I could see losing a lot of time playing a game like this. I only have two suggestions:
- I think the game needs some player feedback on when you're getting injured (like screenflash). It's really easy to just suddenly get yourself killed because you weren't looking directly at your shield/hearts.
- A couple different enemy/weapon types with some rock-paper-scissors style strategy could make the game a little more puzzle-y and deep. Certainly just adding a mechanic or two into a game like this good extend its longevity a lot.