The lack of any music on the main menu threw me off, but I respect the artistic boldness. As I progressed, it became more and more obvious that this was part of the tone you were setting for the mod, and I must say, your execution of setting this eerie tone was superb. I decided to see what would happen if I drown at the beginning, and you kept that consistent eerie tone by keeping the same music and just having that quick "You have drowned" message, and it automatically loading up your save file or starting the game back up. But that was just the beginning.
I really liked your background effect in that cave that used the "First Cave" tileset. You had the foreground tiles, the background tiles blending in nicely, and then the background sky even farther back. It all melded together really well. You did a really good job of giving the mod a sense of familiarity from Cave Story at the beginning, and then slowly transitioning the atmosphere into the mod's unique style by the time you get to the bottom of the island. I really liked the visual effect you had in the dark part of the cave. That was really cool how you drop down and reach "The Bottom of the Island," and the music starts, with the level becoming both more challenging and also having the platforming and background design more defined.
The level design was visually stunning and appropriately challenging. It seemed a bit hard at first, but the strategy of grabbing power crystals whenever you can, closing the gap with the gaudis sooner rather than later, and figuring out how to get those jumps right came pretty naturally; I found myself getting farther and farther each time. By the time that I progressed to the next area, I felt a sense of accomplishment for having overcome a difficult obstacle without having felt any serious feelings of frustration or punishment along the way. That's a very hard balance to achieve, so good job on that.
The one thing that did frustrate me a bit was that once I got past that first major room of the bottom of the island, I was hoping that it would autosave there. I ended up accidentally falling off the edge at one of the later parts, and was a bit perturbed to be set back to the very beginning of the bottom of the island.
I have to emphasize once again how impressed I am with the visuals and level design of this mod. You do a really go job of giving just enough of a hint at secrets that it's not obvious, but that people with a keen eye and a sense of exploration are likely to find it. I found the life capsule and also your secret where you find the cat where it says "Oh. It's just a gato."
That was a cool little area you had with the twin ghost boss fight. I liked the effect of it fading away like Pooh Black once defeated. And I found the cat image at the very end amusing.
The only critiques I can think of are that it might be good to add an extra autosave some time after that first challenging room in the bottom of the island, and that there's this one little part where the visual design of the platform doesn't quite line up with how it behaves in gameplay:
That platform that you need to land on is very easy to get stuck underneath due to the fact that its collision box is very short. If the collision area for that platform was more like its visual appearance in game, it would not only make more sense, but it would make this jump a bit more forgiving, since this was the one jump that I dreaded each time due to the fact that it was hard to pull off.
Those were the only complaints that I could think of.