I've had lots of time in which I might prepare; we haven't played in over a month, and I've been aware that the next adventure would be an assault on Garrett's castle for a great many months.
But all I have is a handful of room ideas and traps, and a dozen monsters I'd like to use. I don't have a floor plan yet or a location of monsters.
My intention is that this should be a fairly site-based adventure, and also a very reactive adventure; raiding the castle will be a matter of teleporting in, fighting vigorously, then teleporting out.
However, this makes it harder to turn a handful of monsters into a dungeon; there's no organizing principle of plot to hang encounters on.
Instead, the natural principle to structure this place is to figure out how the vampires would arrange their quarters to give themselves a comfortable and well-defended life. I'm finding this very difficult, though. I keep thinking that I'll forget something--and I keep questioning earlier thoughts and saying things like "maybe vampires wouldn't have bathrooms in every suite after all..."
I wish I could find a map of a large, sprawling castle. Unfortunately, all the samples I've found are fairly small (which is admittedly realistic, but not what I want). The Stronghold Builder's Guidebook ought to help with this, but its examples are a small keep, an undersea coral castle, a dwarven mining complex, a floating tower, and a multiplanar stronghold--none of which really serve my needs.
Grumble grumble grumble. I hope I can figure out at least enough to keep my players occupied for a long evening.