A tomb of forbidden knowledge? Memories? It feels like a piece of a continuing epic. To me it sounds like you recognized it as familiar (from the other dream, maybe) and no one else found it interesting because it was only a place you had been before. I know I’m not supposed to analyze other peoples’ dreams, but that’s my feeling for it.

I think it’s really cool when a dream is trying to make a symbol out of what you know to be “home” and it decides to collage all of those places that you’ve thought of as your “home” together. So you’ll leave your bedroom from your current house and step into the living room of the house you lived in as a child.

With that in mind, when you said that the building felt like all those different places (castle, library, old house), perhaps it’s not the type of building that was important, but the fact that it was a building/housing of a certain vein or category (mood?) that mattered.