I originally built Castle Nicodemus sometime around 1992. The premise was fairly simple. Take some locations detailed in various sources and combine them together in one place. All the sources had something very specific in common, except for one. That one did have similarities to the others, however.
Why so vague? Because I want my players to guess where I got the pieces. The reward for correctly determining something's provenance? Appropriate and relatively substantial, hopefully.
Coming back to the castle after so long, I first went through and made sure it still worked. I'm pretty sure it does now. I also added 2 new locations, one of which is based on a different sort of source material. Related, certainly, but different.
I'll only lay down one clue. If you're good at recognizing maps, you shouldn't have too much trouble.