Some good castle picks here, and there's absolutely some overlap with final areas given when games like to send players into castles. I'm actually surprised you didn't post more Castlevania Jay. Can't say I'd mind more castlevania showcasing, never played the games myself but the stuff's always sounded great whenever I see it posted.
I also can't help but agree that Lorule is missing a punch to it. It's good, but doesn't quite tickle me in the right spots. Probably doesn't help that I've never played ALBW.
Funny thing with, well, I was going to say Final Fantasy but I guess I really mean Uematsu. People always go completely nuts for him, and when I hear either his older stuff or stuff like the Black Mages I can understand the hype, but his modern stuff just doesn't have the same ZING to it.
As far as music subject suggestions go, any chance of bog standard overworlds getting the spotlight? Tons of stuff to work with in that subject, both Nintendo and Nontendo.
Antagonist themes is another good topic, because no matter the game there's always at least one prick like Pokemon Trainer Blue (except, unfortunately and ironically, modern Pokemon games themselves).
And villain themes is another variant that could be used, some overlap with antagonists but also plenty of room for separation. Plenty of antagonists are just prats rather than villains and plenty of game villains barely even interact with the protagonist. Heck you could probably run several different villain archetypes individually if you wanted to, plenty of variety in there.
Oh, and to tie in with my end-post remarks video game towers could be another theme. Because if there's one thing games love to use as a setting it's giant towers reaching into the sky (also happens to be one of my favourite settings too).
As for castles themselves I'm just gonna leave this here and let it speak for itself. And a very nice changeup for what usually passes for castle themes I'd like to add!
I've seen people posting Zelda castle themes, and Breath of the Wild has been done before (and so has OoT's Ganon's Tower which would have been my pick if, well, I hadn't already done it). So I'll change things up with this chilling topically-appropriate masterpiece.
And why am I the first person posting THIS!!!!???? The song of ruined childhoods and manipulative evil fathers. Pokemon Platinum and Black/White remain the apex of the main series, and N one of the best antagonists. I'm still surprised at how Team Plasma was ever competent enough to build the moving castle though.
In contrast I completely understand why I, the only person to ever play Pokemon Mystery Dungeon Explorers of Time and Darkness (aka the factual best stories in Pokemon ever) am the first to post this beauty. (I mean it's technically not a castle anyway, but I dun care!)
For whatever reason despite being a RPG nut I'm blanking on castle themes beyond this. Strangely enough everything I think of is a tower theme (of which I'm sticking with the mystery dungeon pick), but actual castles? My mind's gone blanker than a wiped slate. Only one I'm deliberately skipping is Valkyrie Profile 2's because it was an atmospheric piece (because the castle had been attacked and the game was about to go full Game of thrones for a while, natch).