I think I might have found the perfect spot to sneak in Vader’s castle!
You can literally grab the silhouette of Vader’s castle from the deleted scene here:
And mask it into the the background of this shot here:
Hal 9000 said:
Oh yeah, and if anyone so inclined wishes to add a distant Vader’s castle to the reverse shot when Kylo picks up and looks at the Wayfinder, I won’t try to stop you.
One can mask this landscape into the background of that shot:
Showing that Kylo Ren’s slaughter happened below Vader’s Castle, perhaps even where Anakin fell at the hands of Obiwan.