Really neat game! Reminds me of bomberman, although the concept is clearly original. I'm glad I played through to the end! There were a couple levels that were especially tough; probably the one that took me longest was the one where you have five bombs you have to place, and then after that one some of them were easier. It might be worth looking into rearranging the levels to create a smoother difficulty curve, but that's not super important and I think overall the difficulty hit the right spot. It would also be helpful to just have a simple screen on the first level saying "stop the lava from reaching the heart" or something to make it immediately clear what the goal is.