the spending hearts mechanic is actually very important. it acts sort of like a countdown
you can't just stay comfortably slaying monsters, you must explore the map and find the xp chests, heart scrolls, and other things to delay the heart mechanic's countdown like nature
although I have to say, without this key information, the game is very frustrating. I was very confused and angry the first games I played till I looked up a guide. they should really give a hint or something to tell players to explore the map