Neat! I'll be honest, I'm probably not the target audience for this game (I thought about it and I've never played a game like the traditional top-down Zelda titles for more than an hour or two), so I didn't expect to actually finish it, but I ended up beating it and had fun along the way! I died a fair bit (maybe a half dozen times?) on account of being a scrub, but there wasn't anything that felt unfair.

I'll echo what's already been said about making the platforming a bit more forgiving and adding some kind of indication as to the boss' health--the latter wouldn't necessarily half to be a health bar, but maybe some visual indication when certain thresholds are passed. Here are my additional suggestions:

• If it's feasible and fits the style you're going for, consider making the enemy pathfinding a bit more intelligent (from what I can tell they just head straight for the player ignoring any obstacles). There were a lot of places where I ended up just cheesing the enemies through a wall which didn't feel like the intended strategy. There was one room towards the end where you mitigated this by adding the little wall-shooter guys on the opposite end of the room to make this strategy more difficult than just engaging normally, so that's another way of dealing with it!
• Adding color-coded doors to the map. Seeing an unexplored path is certainly a clue that there's a locked door there, but it doesn't necessarily tell you whether the path is blocked by a standard key door, a boss key door, or a non-keyed door.
• Making the fire for puzzles non damaging, or arranging them/giving them smaller hitboxes such that it's less easy to bump into them. I think I died at least once from clumsily handling the puzzles (probably on the one where there's a complete square fire fence around the four arrow blocks)...maybe that's just on me needed to working on my jumping skills, but it did feel frustrating to die on a puzzle like that. Again, this might also be something intrinsic to the style that I'm not used to. I know the fire in Binding of Isaac damages you when you walk over it, but you're also never required to do that so
• Along the same lines, it'd be nice if there were a way to manipulate individual arrow blocks besides swinging your sword, or if their hitboxes were smaller to make accidentally triggers less likely. I never quite got the hang of how far I needed to be to only hit one block, so probably half the time manipulating the blocks was spent going, "[slash] Aw man, I accidentally hit two blocks... [slash x7]. Okay, this time I'll get it for sure! [slash] ...dammit."

Keep up the good work! I'm genuinely in awe of anyone who can follow through on any large project like this because I have trouble doing anything that requires more planning and perseverance than writing a few hundred lines of a Python script (and even then, I definitely don't do enough planning...). I'll have to hone my skills on some top-down adventure games so I'll be ready for the full release and not die quite so often and so pitifully.