WebUrbanist has a great
story today about secret rooms and passages. Some of them are well-known, and most are kind of gory (like the secret murder room constructed by H. H. Holmes, which you can read all about in
The Devil in the White City by Erik Larson). The coolest one, I think, is the secret bookcase door at the
Mont Sainte-Odile convent library (see photo above). I definitely need one of these for my dream library!