When I become an incredibly rich technology magnate, perhaps next year, I will craft my palace as Henry Clay Frick did in 1913. There will be bowling. My staff will have their own diner. And the secrets will be many.
In the little montage I've assembled above you catch glimpses of the aforementioned bowling alley (abuts the billard room), the staff diner (completely amazing touch, IMO), and the humungous pre-HVAC heating and cooling system that lived in the basement. All of these rooms were considered slightly "secret" and not necessarily known to the public, although The Gothamist recently got to tour them, and in the link below you can check out their exclusive "secret rooms" gallery.
Frick, if you didn't know was an American industrialist. He was supposed to be on the Titanic, but his wife twisted her ankle, so they lived to go on and construct this East 70th Street mega-mansion in New York City. He also loved art. [The Gothamist]