Cute graphics, fitting music, brilliant sound effects and very atmospheric.
I was very disappointed by the sudden change in gameplay when you reach the purple-brick "burning dungeon" world. Now I like tricky precision platformers, I managed to get zero deaths on BLUE BOY (https://www.newgrounds.com/portal/view/739195 - excellent game) which must be one of the most frustrating such games on Newgrounds, so while I sympathise with players who hate having to go back and repeat themselves, I actually quite liked the lack of checkpoints. You have to treat each screen as a puzzle and "solve" it, learning a set of movements and timings that you can repeat to keep taking you through the solved screens and give you another chance to play around with the one you keep getting stuck on. If you experiment a bit, and pay very close attention to the rhythm enemies move and fire in, all the screens can be solved and muscle-memorised, to the point you can almost do it with your eyes shut. I like that sense of flow. Unfortunately, I think that stops when you reach the burning brick dungeon.
The screens with the big-nosed monsters that shoot fire at you are basically similar to what's gone before. But the ones with the moving flames that skid along the floor are different, because of the way (a) they track the player's position, (b) their starting position depends on where they were the last time you were on that screen, so either where you moved on to the next screen or where you died. It is possible to "herd" them into the position you want. But because you have to pay attention to where you currently are (if you died in your previous attempt at an inconvenient location, for example) it does feel a bit different to other stages of the game. Would probably be helpful if their positions reset each time you entered that screen, otherwise situations arise (particularly if you haven't figured out the herding strategy yet) where it becomes attractive to "sacrifice" yourself in order for them to be more conveniently placed the next time you enter the room, which feels off given the importance of protecting your life in the rest of the game.
There's some pretty tricky jumps required in the last screen that are different to any jumping challenges that have come before. That's okay, and picking the mix of big and small platform-hops that get you through is an interesting challenge, but it's frustrating for that particular section to feel thematically separate from the previous parts of the fire-dungeon - and to have to repeat so much if you die there! Perhaps it would have been better to have a couple of tough jumping scenes with a checkpoint of their own.
I'm glad there was something of an ending scene added as the game is hard work to complete! It feels pretty satisfying to beat it but if it had ended in the "clouds" level, which fits in better with the cuter feel and gameplay of the rest of game, that might actually have been more satisfying than doing so after the tonally different fire level. Just my opinion.
EDIT: thanks for the response! I also thought the death-grunts felt a bit out of place, they're a bit bleak for the rest of the sound-and-feel! Maybe a cute "oh noes" or "owwchie" or just a squeak might be less disspiriting for players who are dying a lot...