The tower is big enough that every Nazgul gets their own floor.
Sauron has several floors at the very top of the tower, mostly occupied by his study and various libraries. These floors notably lack bedrooms, as Sauron has no reason to sleep. Aside from the fact Sauron is literally the top dog and thus should live at the top, he also inhabits this space because the heat he tends to produce would make any floor above his inhospitable.
Angmar occupies the floor directly beneath his master so he can be as physically close to him as possible. This floor actually used to belong to Khamul, but when Angmar moved in he moved Khamul's stuff to the floor beneath his. (He did not ask)
Khamul occupies the floor beneath Angmar's, because it was easier to just inhabit that floor instead of fighting with Angmar for control of the highest room. This was still back when he was trying to get Angmar to like him, you see, so by the time they settled properly into their hatred for each other, it was too late to try to lay claim to his previous space. Oh well, it is not THAT far from his lord's studies. Only an extra few stairs to climb if he wants to assist with paperwork.
Adunaphel occupies the floor beneath Khamul's, and the floor above Akhorahil's. This makes her space a lovely area for the three to get together and have fun playing games.
Akhorahil occupies the floor beneath Adunaphel's. This gives her good access to Adunaphel and Khamul, as well as good access to Uvatha's floor. Since Uvatha has all but vacated his floor in favor of the stables, Akhorahil's stuff has begun to encroach upon his space. He hasn't said anything about it yet, so either he doesn't mind or he has not noticed.
Uvatha has a unique floor, in that despite being close to the middle of the tower, he has his own little exit his boss made specifically for him. Isn't that nice? It lets him walk freely to the horse stables on the backside of the tower, which is where he spends most of his time. Maybe this is why he hasn't noticed Akhorahil's stuff has invaded his space?
Dwar has the largest section of the tower aside from Sauron, having several floors with few windows because he needs plenty of space for all of his dogs. No they can't sleep outside, that's mean.
Hoarmurath occupies the floor beneath Dwar, since she doesn't mind the noise. She picked this floor because Mordor is a nightmare of heat, so somewhere in the middle of the tower is actually where the air is the coldest (The volcanic fissures make the ground floors hot, Sauron keeps the top floors hot). This floor would later be shared with Isildur in the Third Age, and would become Isildur's alone in the Fourth Age.
Indur owns the 2nd to lowest form on a technical level. Yes, this floor is his. No, he doesn't tend to sleep here or really do anything here. Indur spends most of his time on Angmar's floor, following Angmar around.
Ren occupies the lowest floor, because making an old man climb up all those stairs is downright torture. Ren's floor also houses several common areas, including the kitchens and the armory. He enjoys the company these common areas attract.
Back in 2016, Shim Hyung-rae was in talks with the China Real Estate Industry Association to build a theme park based on his films, Reptilian and D-War chief among them. Like D-War II, it never happened (but I dearly wish it did).
