In a large dungeon or megadungeon (such as Gloamhold) you (the GM) can never be perfectly prepared. No one has the time—or I suspect the patience—to prepare hundreds of different encounter areas.
I’ve posted a tidied up and re-organised edition of my old post The Illusion of Detail to Gloamhold.com.
Check it out and let me know how you add the illusion of detail into your dungeons!
