First of all congratulations, even as a demo so far this is fantastic to play. I would especially like to complement your use of colour palette very reminiscent of OG Castlevania, everything is easy to identify what I can/cannot stand upon, my characters boundaries are well defined and the animations when firing especially are amazing.
It can be a bit hard to hit flying enemies without the diagonal shots and I know you're already implementing that, however just being able to shoot upwards would be a big help. I do have a mild critique for enemy placement though. when you pickup the first power up and the map changes you drop down and are immediately faced with the green tentacle creature, usually its good practice to allow a player to experiment in a 2d environment with what a creature can do before you engage it, the problem is once you drop from the platform onto the bookcase you have two choices, drop down take a shot at it without knowing what it can do, or drop down through the next platform observing what it does to attack then jump up round the back of it and kill it. But I'm not sure as of yet if you'll get these enemies to track us and turn if we do that. I would advise clearing away some of the red wall flesh and putting this enemy on the farthest bookshelf. That way a player can drop, see the projectile coming and jump back up to avoid it.
seriously though apart from that one tiny critique this game is amazing so far cant wait to play the full version definitely scratches all the right itches for metroidvania and the soundtrack had me hooked from the beginning very alone in the dark.