There are 3 Secret Buildings in Billionaire that can all be unlocked at the same time. In order to unlock these buildings, you're going to need to have a lot of money, and time, at your disposal. You need to have upgraded these 7 buildings, all the way to level 50. The buildings are Magic School, Dinobot Museum, Imperial Star, Avalon, Eldorado, Mountain of the Eye and Underwater City. Once you do that, you will, without any notification, unlock these three buildings. They are not free to purchase, so you're going to have to shell out a bit of money for them. Their earning rates are roughly (1/10) - (1/5) that of Asgard, so they are really only useful for collectors purposes.
If you know how to unlock Colossus Citadel or Deity Dimension let us know in the comments below!