The game physics are solid, it takes a little bit to get accustomed but definitely work, jumping, attacking, using the trampoline and the rope feels very NES like and intuitive, although the knife guy has a hard time landing a attack, maybe give him a sword or spear haha.
the looks or this game are nice, pixelated but not over-simplified, the boss looks specially cool; aside from that I would love to see more upgrades to the weapons, I am guessing they work similar to how we have Holy Water [I] [II] [III] in Castlevania games, that appears to be appropriate for this kind of game.
I understand the lack of music because this is a demo version but I am sure this game would benefit from a hype song to start the adventure. And also consider making the try again screen skippable so the player can die faster.
Keep the good work!