@Lisa487 me too I'm confused.
I would to add boring legal advices. 😁Make sure everything is in order and that you can rent all (apartment and room) in Airbnb according to your local rules. Check if people can sleep in that room and what you can do there, in certain areas if it is classified as a cellar it must be used only for storage of goods. I guess you have a window. I suppose the stairs are common to other apartments. Will the neighbors accept your guests use them often? Will you have the building's permission to have short rentals?
You have to imagine what kind of guests you will have or want to attract. Size of the room, how you can reach and budget are very important.
A small office with a desk, printer etc can be very useful. A games room with TV, video games or a cinema room with large screen and stereo system. If you don't allow smoking that could be the smoking room you prepare with suitable materials. An extra bed is always useful, a sofa bed can be a good idea. If the stairs are private it can be the second bedroom, not perfect but useful. You can mix all: smoking room with sofa bed, TV, video games, stereo, a desk, chair, and a printer.
Specify well in the ad how to reach it, via private or condominium stairs and every details.
Good luck!
p.s. if big enough, I would do my best to bring the plumbing, even asking the condominium for permissions