1. map size: less is more. you can also focus on room to room challenge as well as tighter, better areas as opposed to spread out ones.
2. make the "big steel door" the center point of the dungeon(area) all major paths converge onto. at most, 3 paths, though preferably two. one, a combat trial, one a platforming trial, the third, a boss guarding the shaft itself. To stop platforming from getting frustrating, avoid pixel perfect, or overly precise timing and jumps.
once a challenge is done the room should loop back to the main entrance to avoid running a gauntlet again, backwards.
your game is fine. its one of the better H platformers.