I felt like the game was asking for pixel perfect action, but the controls did not give me the ability to execute those actions.
This is how I felt through the entire game. I persevered and finished it, but I sure died a lot because the controls were inconsistent.
The two or three rooms that require jumping just onto the edge of a block to avoid spikes (while simultaneously attempting to sync with the purple blocks) are extremely frustrating. The jumps require precision but the player moves horizontally too quickly to allow the necessary control. The result tends toward a luck-based outcome rather than skill-based.
The other problem with control is the inconsistency of jumping. From what I can tell, it seems you can either jump while standing still or jump after running a bit, but not right when you start running. Or at least this is how it felt; I just know there were times I pressed the button to jump and nothing happened. This resulted in many instances of running headlong into pits of lava or spikes.
All of that said, I was eventually able to get a feel for the controls and play around their inconsistency.
I like the graphic style you've got going and the general presentation of everything. The colors work well also; most things are quite readable. It wasn't immediately obvious that the candles could be broken, but perhaps that was intentional.
In terms of level design, I think you've done a very good job with a couple exceptions. As mentioned already, the beginning is a bit tedious because nothing really happens. I think one room with a ladder and blocks to initially convey how to climb and jump is really all that is necessary. Also, the fall from the ladder of one room onto the spikes in the room below is a bit of a cheap shot. There are also some areas near the end where it seems impossible to get through without losing at least one heart. But it may be that I just didn't play those areas enough. Otherwise, the rooms compelled me to keep going (frustrating as that was, at times) and gave a real sense of accomplishment when reaching the next checkpoint. I particularly liked that some rooms had to be gone through a second time but in the opposite direction. I also liked the slight puzzle element to at least one room (the dark one with a ladder, breakable block, and purple ball moving horizontally in the crossing corridor). The variety of room types was good, though there may have been a few too many "spikes and purple blocks" rooms.
For the most part I like the enemies, but feel there should have been one more mobile critter in addition to the bat and snake. I agree with earlier posts that it feels wrong that the bats can't be hit except when flying. However, I think it results in better gameplay because it requires more thought. You can't just sit back and shoot it; you have to either run under it or get close enough while it's flying to hit it and in both cases risk getting hit. In general, the dynamic of that decision would benefit from an incentive to fight enemies for the loot they drop. From what I can tell, there is no way to spend it.
I really liked the bosses. I didn't seem to have trouble with the first like others have. There is one block that if you stand on in the right spot makes the fight extremely easy. It is close enough that you don't have to move for your weapon to hit the boss and the vertical beam misses every time. Also while standing on this block, only one of the thrown blades can hit. So it's easy to simply blast the boss with your weapon while occasionally jumping to dodge a thrown blade. The second boss, in my opinion, was the most difficult but was also my favorite. I don't know if it's intentional, but the player takes no damage from touching the boss in bat form. The last boss wasn't a cakewalk but wasn't too difficult, either. The falling fire doesn't seem to reach the very back of the room, so I was able to lay into the boss, jump back for a bit, and repeat.
I was kind of surprised when the game ended after that boss fight because I was really getting into things at that point. Not that I don't appreciate the work you've put into this, but I think it would be great if expanded and the controls improved.
On the technical side of things, the game was a bit sluggish at times. Near the end I started getting slow down for some reason. Not enough to make it unplayable, but enough to be noticeable. There was one instance where I was hit by a monster and got teleported to the far left side of the room. I was leaving the room on the right side just as I was hit, so I wonder if the coordinates got updated for the room I was going into but the room hadn't yet changed? There was also one time I died while the screen was shaking and the screen kept shaking when I restarted at the checkpoint. Dying again did not stop the shaking; I had to quit and restart.
And one final thing: I don't agree with those saying the checkpoints should be closer together. For the most part I think they're fine as they are. It's mainly the controls causing difficulties, not the checkpoint locations. I will agree, though, that it's annoying to sometimes have to go through two very difficult rooms to reach a boss after restarting from a checkpoint (I'm referring to the second boss, here). Overall, I like that the checkpoints were spread out because it gave me a sense of accomplishment when finally getting through a section.
Thanks for the game! I did enjoy it despite the frustration.