Aside from their function as currency, hearts are also your source of experience in Castlevania 2. Here's the breakdown of their monetary and experience value.
Heart Monetary value Exp. value
2 hearts 1 exp. point
4 hearts 3 exp. points
6 hearts 5 exp. points
You start the game at Level 0. After gaining a level, the experience counter goes back to 0. Level 6 is the maximum; you will not be able to gain any more experience after reaching it. Also, after gaining levels, hearts gained from enemies in certain locations (mostly easier areas) will no longer give you any experience. A good guideline seems to be to gain one level in each of the Mansions, and two in the final Mansion.
So what exactly do levels do for you? Here's a detailed breakdown.
Level Exp. needed Life Damage taken
0 0 12 100%
1 100 12 75%
2 150 16 50%
3 200 16 25%
4 250 20 12.5%
5 300 20 7.14%
6 350 24 6.25%
Remember that whenever you use a continue, your heart and experience counter will both be reset to 0. So it's in your best interest to try and stay alive. (duh
Especially if you're going for the ending that requires you to finish the game within 8 game days. In that case, it's also a good idea to spend time levelling up in the mansions, since time stops when you're inside them.