This gave me real old school Nintendo vibes. The music, the graphics, the controls. I actually got frustrated with it the first day of voting, and decided to come back when I had more time and wasn't so frustrated. The second playthrough was better. You had a massive amount of content, and I barely scratched the surface. Pretty amazing in 30 days. Your game page was well-documented and reminded me of the little manuals that used to come with games. That was nostalgic. Also loved the map. It was needed.
I ran into a number of issues, some of which I would probably be less picky about if the game hadn't looked so complete. First, I got soft-locked into a room. While going down a vertical corridor, I went right through a red door without destroying it. On the other side I was stuck in a room and could not destroy the red door from the left side. I had to quit and restart. Not so bad because it was right below a save point.
The collisions for slimes that you had to jump to hit were not clear. The sword would swing, but the slime would not take the hit even though the sword passed through it. You had to be at the same vertical level as the slime at the same point that the sword was at the bottom of its arc for damage to be dealt.
There was a lot of reading necessary outside the game to play the game. I was glad you had the manual, but I had all these buttons on my controller and none of them would shoot a fireball. So I went back to the page and found it was Up + B. Even in Metroid they told you to press Select to switch from your gun to missiles. I would expect a game this polished to tell me how to shoot the fireball in-game when I got it. (Admittedly this isn't something I would expect in a less complete game.)
The level design was pretty good, but it missed some design elements I would expect in a game this polished. It took a long time to get to the fireball, and it was not a clear route. Typically the first upgrade is close by and introduces a mechanic, then lets you play with it. The map expands as you play, and things you passed by become clearer. This felt like being thrown into a dark room and wandering aimlessly for a while. That's what made me frustrated the first time. I didn't even know if I was going the right way. The second time I just kept hitting dead ends and dying on spikes or in blood vats and just kinda stopped playing. If you were going for a hard game, well done. Just wasn't my cuppa.
I also agree with @netpro2k about the map. I wanted it to be on the Select key, which instead just gave me a negative beep sound. Also B didn't exit the menu, which again I expect things like that in a game this polished.
My last comment is that the movement and jumping felt like an old school NES game. That was nostalgic, but not fun. I always found those games to be very finicky about jumping. This one was no different. The movement was slower than I would like, but not unexpected for what the game looked like - which was an old NES game. Jumping was very particular, and I did get the hang of it, but it didn't feel natural. Compared to old NES games, it was very faithful. Compared to modern games, it was less than what I would expect and compounded the frustration of playing the game.
Overall, really good job capturing the aesthetic and making a solid NES game. It looked, felt, and sounded like one.