Question: 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.

Answer: 
**Orcs**

From *The Two Towers*, Book III, Chapter 9, "Flotsam and Jetsam":

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> 'All this about the Orcs of Barad-dûr, Lugbúrz as they call it, makes me uneasy,' said Aragorn.
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The books are full of other references, particularly to the name Lugbúrz, and it's clear that in addition to Orcs the high command of Sauron's forces also dwell there. For example, from the earlier chapter "The Uruk-hai":

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> 'You seem to know a lot,' said Ugluk. 'More than is good for you, I guess. Perhaps those in Lugbúrz might wonder how, and why.'
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As a small addition to Victim of Circumstance's answer, from [thelandofshadow.com](http://www.thelandofshadow.com/barad-dur-history/):

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> The tower was full of treasure, weapons, armor, materials, great stores, **barracks for the footsoldiers** of the tower (on the lower levels), and rooms for **high-ranking captains like the Mouth of Sauron**.
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From [tolkiengateway.com](https://tolkiengateway.net/wiki/Barad-d%C3%BBr):

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> Around T.A. 1050 Sauron returned and secretly made a stronghold at Dol Guldur.[3] Gandalf did not discover that Sauron was the master of Dol Guldur until T.A. 2850, and in T.A. 2941 the White Council attacked the fortress. Sauron retreated to Barad-dûr, which the Nazgûl had prepared for him, and began to rebuild it in **T.A. 2951. From then on Sauron stayed in Barad-dûr** and conducted his war on the free people of Middle-earth from there.
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