The art in this game is really nice. Great work nailing the atmosphere. I think you accidentally forgot to include shiori's sprite in the boss battle (she appears as a big black square to me), but otherwise it was really amazing. Like someone else said, the collision gets in the way a little but. I found it unclear what walls and ceilings had collision a lot of the time, which would make it so I ended up bonking my head into some ceilings. The black sparkles for floors are a great idea, but I feel like they blend in a bit too much to the dark backgrounds you have. I'd probably change the color to white, red, or blue to match ERB's color scheme, and then for both the platform markers and hidden treasure markers, increase the number of sparkles so they stand out.
The underground stage also has some segments where you can fall into ink, but the camera doesn't show it, and since you end up going down a lot in that stage, I ended up dying a few times because I missed platforms that I couldn't see.
For combat, I think maybe something simple like having enemies flash red or white when they get hit would be huge. Some knockback on the big enemies that take 2 swings would also be nice, but that's not super necessary.
Again, this game was very good and I was really impressed with the level of art you have here and the atmosphere you guys created. Great job!
