What. A. Gem. Really glad you stopped by and therefore lead me here - I really had a blast from the past. It's really strange how restrictions (like the movement here for example) can enhance the game experience - I was really on the edge of my seat. perfect atmospheric sound did a lot as well.
sidenote: I think you could even lean more into that retro feeling with your visuals by purposefully rendering into a lower resolution and limiting the color palette (down to 1bit even, maybe?), making it almost look like the fake 3D these early games used to draw these dungeons... Just saying it could do a lot for your game with only a little bit of experimenting.
BTW: the door seems to be openable right from the start, if you just walk through ;-)