I beat the demo! Didn't think I would be able to. Honestly I struggle with these "oldschool-difficult" precision platformers, however I found it very rewarding to make it through the rooms a bit faster after I died enough times, and getting the health upgrade was definitely a relief and motivated me to keep going.
There's a bit of input delay on the sword which really messes me up, I'm not sure if that's by accident or if it's meant to be a callback to Castlevania but I feel like it detracted from the experience, although combat was a lower priority than platforming.
Using items like bombs feels quite finicky, and since they appeared so rarely I wasn't able to get the hang of using them so that I could blow up all the walls to find secret goodies, which feels a bit punishing - if that's the intended feel for the game, then fair enough.
Overall I think this game is great for what it's aiming to be, with a strong visual style that makes the most of its low-resolution graphics and communicates to the player where to go with no explicit signposting, and delivers the game's intent very clearly. I may not be able to beat the full game if it comes out, but I hope you'll be able to finish it, and be proud of it!