I've cleared most of the rooms now, and I have a small key. I have yet to:
Reach the Third Floor,
Reach the middle room, and the right column of the Second Floor,
Reach the Midboss room and the item he's obviously keeping (and the rooms north and east of that item's room) on the First Floor,
And obviously, I have not reached the Boss Room and the rooms north and south of it in the Basement.
There's also two chests I can't reach: One is on the first floor, on the top-right door of the entrance room, and one is in the Basement, two doors north of the Boss Room.