A. Vader's Castle, aka Bast Castle, aka Vader's Fortress, was located on the planet Vjun. It was one of the few places Lord Vader could recuperate outside of his armor. After his death, the reborn Emperor used it as a staging point for his return to power. His Dark Jedi made it a base for their operations. Luke Skywalker and some of his Jedi eventually find it and are able to defeat the Dark Jedi. Vjun and Vader's Castle were featured in the game Jedi Knight: Jedi Academy. Jedi Jaden Korr and Kyle Katarn go to Vjun to retrieve a lost apprentice. (I'm really not quite sure what you want for EU relevance and the game.)
Q. Identify the "A-Wing Slash" and its first appearance in the EU.