Var ligger Vader fästning?

Per Pablo Hidalgo (vem är Mr Disney Canon ), det är egentligen Mustafar

if it's the one I think it is, it's the one from Ep III.

Per Wookiepedia (som citerar kanonfilmerna och andra kanonkällor)

After the end of the Clone Wars, the Galactic Empire took control of Mustafar and maintained secret facilities on the planet where Jedi survivors of Order 66 were interrogated and executed. Rumor had it that Darth Vader carried out these interrogations, torturing the Jedi for any useful information before their execution. Additionally, both Vader and Sidious often returned to the planet, using it as a focus for meditation on the power of the dark side of the Force. Four years before the Battle of Yavin, an Imperial fleet was stationed above Mustafar commanded by Grand Moff Wilhuff Tarkin aboard his Imperial I-class Star Destroyer the Sovereign. During a mission to rescue Kanan Jarrus, the Sovereign was destroyed during a duel between Jarrus and the Grand Inquisitor when his lightsaber was cut in two by Jarrus and fell into the ship's power core, sending the vessel falling towards the lava fields below

In 0 BBY, Darth Vader had a personal abode on the very planet where he had been disfigured. Shortly after an insurgency on the desert moon of Jedha, Vader was in a bacta tank, out of his armor, when his servant Vaneé admitted Director Orson Krennic, who wanted to discuss the workings of the top secret Death Star.

Enligt filmförkortning är det Mustafar:

His shuttle pitched and rolled as it entered the atmosphere, riding rivers of black clouds tossed by howling winds. The shuttle’s stabilizers and gravity units maintained an interior equilibrium, but Krennic found the experience no less unsettling for the relative lack of discomfort. He grasped his chair tight, kept his chin raised, and reviewed for the sixth time everything he knew about Darth Vader, recounted every one of a dozen tactics he might use in conference with the man who had made Mustafar his base of operations. (Chapter 13)

Filmens junior romanisering indikerar att Vaders fästning finns på Mustafar , planeten där han och Obi-Wan slog sin ikoniska kamp i slutet av Star Wars: Revenge of the Sith.

When Krennic arrived on Mustafar, he’d ordered his pilot to land next to Vader’s monolith, the tall black tower the Sith Lord called home. He’d left his death troopers inside the shuttle and entered the forbidding place alone.

Rogue One: A Star Wars Story – A Junior Novel

Om varför väljer Vader att bo här, det svaras av Rogue One: The Ultimate Visual Guide faktabok. Kort sagt befallde kejsaren honom själv att basera sig där.

Dark Monolith
Vader's personal abode is an obsidian tower on an inhospitable world. It is by the Emperor's design that Vader lives in such an unforgiving environment ... The stark, modern structure is built over an ancient castle full of dark secrets.

Vi kan bara övertyga vad kejsarens motiv var, men det är verkligen i överensstämmelse med Palpatines vridna sinne för att få Vader att leva i den plats där han led sitt största nederlag.