Castles with dead spaces, forgotten/secret rooms and passageways...

http://www.atlasobscura.com/places/secret-passages-of-mont-sainte-odile

http://www.dailymail.co.uk/news/article-2037644/Victorian-kitchen-remained-untouched-60-years.html

It's really too bad that more information isn't available for this one:
https://en.wikipedia.org/wiki/National_Library_of_India