By now it's no longer a secret that Rogue One: A Star Wars Story is absolutely visually gorgeous, as the film is clearly jam packed with awesome set pieces and beautiful planets. Even the most terrifying locales look genuinly amazing. With that in mind, one of the most fascinating locations in the entire film is Darth Vader's castle on a lava-covered planet, and according to Star Wars designer Doug Chiang, it was specifically centered around Vader's relationship with his bacta tank. Chiang explained:
Vader's castle was really fun. When I knew that we were going to possibly go back to Vader's castle, I loved that idea of, "Okay, let's establish Vader's home." The initial idea was, "Why would Vader live here?" In our minds, we started to come up with a little bit of a backstory. That perhaps this place had special meaning for him, and that this is where he comes to meditate and to heal himself. We started with the idea that maybe it should be built around his bacta tank chamber, and he comes back here to rejuvenate himself and also to meditate. So from there, the structure itself grew out from the bacta tank, and there were certain ideas that we tried. We were trying to go for a very iconic shape, and we always love tall towers. Ralph McQuarrie actually drew quite a few small thumbnail sketches [of Vader's castle] that were very intriguing. They were kind of angular versions of a tower, and I saw the potential of where he was going, and I just exaggerated that quite a bit.
If you ever want a definitive look into the mind of a genuine creative, then that StarWars.com interview should give you a solid understanding of how the process works. It's not terribly surprising to learn that Darth Vader's bacta tank served as the starting point for Doug Chiang and Ralph McQuarrie when creating the Sith lord's castle. The tank is one of the main focal points of the sequence on the hellish planet, and it seems to be the most fundamental and functional aspect of the gothic edifice. They started with that specific chamber, and then moved outward in order to construct an imposing castle that would match the personality and iconography of the legendary villain. By contrasting it sharply with Jedha's white tower of the same design, they were able to make it a place of refuge and reflection for the fallen Jedi.
Despite the inherent challenges in making Darth Vader's home castle, there was still plenty of established precedent in the greater Star Wars lore for the team to draw from. It has already been confirmed that the castle is located on the planet Mustafar (the location where Vader received his terrible injuries in Revenge of the Sith) and several pieces of concept art from Return of the Jedi have suggested that Vader's castle was actually supposed to show up in the Star Wars films much sooner. The team obviously had quite a bit of leeway when it came to creating this location, but they also certainly had a solid foundation to start with.
All of that hard work seems to have definitely paid off. Darth Vader's castle in Rogue One is a perfect representation of that character at that stage in the saga's story, and it definitely adds new layers to the underlying Star Wars mythos. Let's keep our fingers crossed that future anthology films explore the location even further.