(spoilers)
Neat game.
In my opinion, the hearts should be replaced with "attack points" or something like that. Because right now, at first I thought it's better to save as much health as possible, only later I realized its actually better to level up AFTER you've spent all your HP on monsters, since the gold dropped by monsters can have an overflow. So calling them attack points would make more sense - monster number is how many attack points to kill them, if you have less than that number - you die. And replace heart pickups with AP potions or whatever.