Just put it on Steam already you animals.
Incredible pixel art all around but particularly for Merlin, wonderful chiptune remix score, good level design with signposting for the player, excellent intro cinematic to set the stakes, I could probably go on.
You're likely all-too-aware of the little graphical glitches here and there so I won't harp on that. I would say though that I think you should err more on being too generous with your player. Difficulty is pretty high here and rooms can be huge, making death really hurt. If boxes could "checkpoint" and stay put if they were more-or-less in the right area, this would be an incredible boon and take a lot of the sting out of dying. And I think the box scaling killing Merlin instantly should be reworked, it's just too frustrating and common and doesn't really add anything to the experience. Scaling boxes could hurt enemies though, that'd be fine.
Give Merlin a whole set of spells, flesh out the castle with lore and a deeper aesthetic and I think you're well on your way to something players on Steam would pay money for. Great work.