@Mbun Oh man, thanks for sharing this one. I love the puzzle-y aspects to the layout, I can pretty much assure you I never found my way into that pipe entrance up there myself either, nor would I have a change of getting the secret music box in the extra room. The payoff for exploring in this zone is legit.
Those breakable blocks are really enticing, but having found them I don't know if I would have thought to look all the way up in the air back at the beginning of the zone. The pathway in is REALLY hidden.
I love looking at Super Mario 3 courses, so many of them are SO compact. That's almost always a good thing, keeping levels short, especially if you're asking players to linger over them and explore every nook and cranny like 5-1 here.