It's a good start, love the minimal graphics and great sound effects too! The graphics feel like if dark souls were made on an old computer system, it is a really neat style! It's readable as well, I had no problems understanding what I could fall down through or walk through, and what was the level bottom/walls. I want to see more!
The music feels a little out of place, I would expect chiptune-like horror, but it sounds like it's instrumental wave based instead. It still works for atmosphere, it just stuck out to me given the rest of the retro style presentation.
The way attack works makes it hard to hit enemies in the air or when coming down from an upper platform. I imagine this vertical handicap will only get worse when/if the later levels are more vertical in nature. I think it would feel much better if the attack animation damaged a larger vertical area, or was extended (cheated just a bit) in vertical hit length. You can keep the bottom "safe zone" in the player attack to leave the possibility of short enemies, but there were several times I felt the attack should have hit when falling/jumping by a frog/bird and instead I was punished with a damage hit on myself.
Being surrounded by enemies on the ground appears to be death, pretty much for the same reason. The attack animation has no backwards or overlapping damage with the player area itself, so once an enemy touches the player they take damage even if they are mid-attack. While this is fair (the whip should technically only damage on strike after all), it still feels bad when fighting two surrounding enemies at once for example. I would love for the first frame of the whip drawback to have a hit area on the backside of the player too. It wouldn't be too practical for fighting but would help alleviate being surrounded or tag teamed from both sides since the player could time hitting the "first" enemy with the backswing, then hit the later enemy on the front swing as normal. This advice comes from my heavy playtimes of the NES castlevania games, so do what you wish!
It needs a way to earn health back with good play. I understand not having enemies drop health, as that would feel too cheap and add an element of luck. But maybe something like a combo system that drops health pickups to reward the player for killing enemies within a combo timer by restoring some health. Or add a health pickup to discover in the level for exploring a bit off the "fastest" path or attacking hidden blocks. (I was actually expecting that to be the reward for taking the top route on the triple ledge mountain, as the safe route through that section is to ignore the frogs completely.)
My main strategy when seeing new enemies on screen is to hold back and take the enemy out one at a time. While this works, it isn't as fun as if the game encouraged me to have a "flow" through the enemies/level design. You have a few patterns of enemies like that frog with a walking enemy, that if timed and charged at can be taken together with a single well placed attack. I would love to see more of that, as it feels good to figure those tricks out with each playthrough! I think having a health recovery reward or checkpoint system could also help encourage the player to press forwards instead of play it safe, creating a great feedback loop for improving the player's skill at the levels.
The difficulty here feels just right to me. If not for my problems doing jump attacks I could clear this with a few more runs. This is a fair feeling level distance to go before fighting a boss, and I did want to retry till I could get there reliably with health. I only didn't beat the boss cause I got tired by the time I could run the level perfectly. In a bigger game I would come back to try and beat him to see what's next.
However, I would not want to see any longer distance between a save/start point besides what is shown here. If levels get longer or areas start branching, you will need more checkpoints or a way to restore health before the boss fights when learning them. I think with a lot of practice I could do better of course. But I would expect a longer game to feel about this level's length between checkpoints, just ramping up the difficulty of each section instead of the length.
The boss first run is almost always going to be to die to him due to not knowing his patterns and barely clearing the stage, so a checkpoint somewhere before him would respect your player's time investment. You could still put some level area between the checkpoint and boss if you feel the level section would help practice attacking or break up the runs a bit.
I had a lot of fun playing! I hope this feedback helps should you expand on this idea! Thanks for making and sharing this! :)