I'm sure this has been mentioned before, but it bears repeating.
Each time you take the elevator to the next stage, it goes down instead of up. This means that, logically speaking, the Firestorm Citadel is actually in reverse, with Blackstone Bridge at the top and everything past that being a giant basement.
Doesn't seem like it would look too imposing from the get-go. Maybe there are other floors above Blackstone?
Vanaduke's boss room doesn't have a throne.
Maybe he left the true throne room intact by turning his arena into a makeshift one.
I would enjoy a Vanaduke rematch, where he gets up from a shining, ivory throne and it suddenly turns to ash and crumbles to the floor, smoldering.
Anything to be learnt from the Shadowplay mission, perhaps? I'm sure most people only played it once, might be worth re-visiting for additional lore elements.
What we know is this:
-Vanaduke is now gone for the time being
-Undead, instead of Fiends, have taken over most of the Citadel's remains, most notably the corpses of Almirian soldiers.
-Furthering that, Almirian soldiers are still present either as ghosts or zombies, even with the disappearance of their king.
-The ghost of Torin is stuck in the past of the kingdom, and does not notice the zombie-soldiers.
-The Citadel shuffles itself now, being freed from the gap in time Vanaduke had created.
-Machinery such as the Wheel Launchers still function automatically, giving the impression that they're either controlled by ghosts or function that way in the first place.
-The fire that keeps Slag Walkers and Howlitzers alight can't be put out by rain or even sprite-water, giving the impression that there's still some force holding them together.
-The Throne Room is still fiery as ever, giving the impression that Vanaduke's influence is still present in that particular area even after his defeat at the hands of the Spiral Order.
And of course, the Whispering Venom and friends.
Overall, the Citadel shuffles itself currently, but before Shadowplay it was frozen and unmoving. Leading to the question of if it really is a Citadel at all anymore, or if it ever really was to begin with.
Well, it's buried in the Clockworks. Though I don't know why Vanaduke would just be sitting in the basement. Maybe its the vault. Maybe he thinks that the intruders are their to steal his family heirloom in the vault, which is burned to a crisp and he doesn't know it.
Whatever, it probably is a citadel and the Spiral Order can't break through the walls near the bottom floor instead of sending squads to go through literally hell.