Door a) The cracked door between Elio’s and Oliver’s rooms
This door does a fine job It has many purposes.
You can use it to spy on the object of your affections and watch them sleep.
You could see anything they were doing in bed, actually, because there’s a perfect view of the whole thing. Oliver basically has zero privacy. I’ll leave you to draw your own conclusions about what else Elio might have seen. Or Oliver, I suppose, because you must be able to see pretty much all of Elio’s room from Oliver’s side of the door.
You can also use this door to see the silhouette of the object of your desire after a very prematurely terminated blow job.