This is a BBEG dungeon waiting to happen.
I think this is probably one of the easiest prompts while at the same time one of the most challenging. This is the sort of thing I would expect to find either midway or toward the end of a campaign. The Dark Castle is normally home to monsters and villains of untold power.
“Dark Castle” conjures up a gigantic structure, a medieval castle of immense proportion. Its construction is probably mostly an obsidian block or possibly magically enchanted stone. It resembles a regular English, German, or Austrian castle from ages past in the real world, only surrounded by storms almost constantly. Its shiny black exterior is only illuminated by flashes of lightning, making the flags on the pinnacles almost unreadable.
Such a castle is likely the home to a dreadful Wizard, Knight Lord of Chaos, Greater Demon Lord, or Lich of immense power. The guards are likely to be heavily armored and heavily armed, possibly with magic. Within its corridors lurk all sorts of nasties- demons, undead, and humanoids warped by foul magicks. There could even be a dragon lurking within or nearby.
Mapping castles: the ultimate DM challenge.
Mapping out castles is more difficult than building any dungeon. Real life historical castles were enormous. Yes, floorplans of these huge stone structures are available online and in libraries. (Gasp! IKR? Like, big buildings that hold books in the real world.) But real world castles didn’t house wizards, demons, and dragons. And logistical matters? Pffft. Yay magic!
My advice to DMs is to either take advantages of RPG sourcebooks with castle maps and then kitbash in your own rooms as necessary, or draw some modular rooms for towers, quarters, chambers, etc and then design any special focus areas for specific encounters. Both options turn the castle into a sort of point crawl, but it will save the DM time instead of drawing out a massive nine level poster map-sized castle.
Don’t get me wrong, constructing and mapping castles can be a lot of fun. But, when you’re eyeballs-deep in prepping for your next game session, on top of work, school, kids, family, and social stuff, do you really have the time? Another option would be to delegate it to a player who does maybe have the time or even a friend who isn’t in the game who just loves doing detailed maps.
*Friends? Social life? Yeah, not me. Maybe you do. I dunno.
Thanks for stopping by today. This was fun. So much more to come yet this month and most of next catching up on this and #Monstober.
Jeff's Game Box 
I found cadging maps from books and other sources invaluable over the years and another source I can reccomend is Pinterest. This has the added avantage of being able to copy and paste into a document rather than having to re-draw/scan.
YMMV
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