Barad-dûr was a stupendously tall tower in Mordor and home to the Dark Lord Sauron.
Apart from the big man himself, who actually lived there, though? It's hard to imagine such a huge building lying largely empty. Were there orcs who were assigned to stay there? I'd include people who worked in Barad-dûr or those who stayed there from time to time as well as any permanent residents. I have no specific time period in mind.