As children, many of us are drawn into architecture by the idea of secret passageways and hidden spaces. In the real world, 'secret' passageways can easily be incorporated and actually help streamline a the design of a room. Hidden doors can be a big draw for grown ups too!
Through the use of cleverly designed wall paneling, doors can be seamlessly integrated without anyone being the wiser. To make it work, be sure to use industrial touch latches so that handles aren't needed, and 'invisible' 3-way hinges for a clean opening.
(Images: As linked above)