Sometimes even a bad book has a good idea. One of them is the book someone must have found for me at a library sale or garage sale, "The World's Most Mysterious Castles."
It's not a bad book, but not a great one either. It's supposedly about mysterious castles. It's more about "mysterious*" than about "castles." So while it does tell the history of some castles - and the associated hauntings, mysteries, and oddness - it tends to veer off into unrelated history, battles, weird speculation, and so on. It also tends to substitute wild speculation for actual mysteries.
That said, there were two bits that made me think, "this might be good in a game." Here they both are:
The Cryptic Message - In one castle, there was a a carved gravestone with a cryptic message about a hidden treasure. Someone carefully and thoroughly defaced the message to illegibility . . . but before the vandal came along an antiquarian copied it.
So you find what's clearly part of a treasure clue, but now you have to truck around to sages and private libraries and hidden wizard-only bookstores to find the rest of the clue. It's a good reversal of the usual "find the map, then find the spot on the map." You've found the spot the clue refers to, and that there was a clue, but not enough to go on.
This kind of stuff can explain a long-lost dungeon entrance, too - everyone knows the place, but centuries of would-be looters can't figure out the way in now that some predecessor destroyed the clue.
Non-Ray Shooting, Non-Talking Giant Stone Heads - The book makes the suggestion that the giant stone heads (Olmec, Easter Island, Greyhawk, your choice) might be a form of fortress. They could be a psychic focus, making a supernatural defense screen for an area.
Obviously they still need to be enigmatic, talking, and able to shoot death rays from their eyes. Duh. But also, psychic castle! Maybe it's proof versus psis, or proof versus everyone but psis (who can attune to its frequencies and penetrate to the treasures within.) Maybe it's a psychic prison, holding ghosts or tentacle-faced monstrosities inside.
That's about it - the rest of the book is so boring I won't even link to it. But I got two ideas out of it, so it wasn't totally wasted time. Enjoy!
* "Ahh, there's no such thing as mysterious."