Camouflaged secret passage at Cu Chi Tunnels in VietNam. These images are Public Domain images.

Although secret passages have been glorified through history in fiction and lore, they really did exist and served a variety of functions, mostly as ways to escape or hide during enemy sieges. Most secret passages in castles were hidden from view or camouflaged in walls, staircases, behind bookshelves, or in closets. Some were simple trap doors hidden in floor boards or under rugs. While some were built into the original plans of castles, others, particularly in jails, were tunnels dug into the walls or floors leading to escape.

Secret passages were common features of Egyptian pyramids which were used to access the burial chambers and to trick would-be marauders to enter booby-trapped rooms. Later in history, Christian priests used secret rooms to worship and elude persecution from Catholics. In the 1500s and 1600s, Catholic priests used passages known as "Priest Holes" to escape persecution from the Protestants. Old Catholic families would often hold services in secluded and specially designed parts of their houses such as in the attic or roof. A Priest Hole was often built into one of these sections of the house (sometimes in its chimney, behind wall panels, or under floor boards) and was nothing more than a small, hidden enclosure in which the priest could quickly hide along with the sacred vessels, vestments, and alter furniture. Sometimes, these "holes" were so small that the priest suffocated. The existence of these hidden areas were well known to Protestant bounty hunters, but were often so well-built and disguised, that not even expert masons or carpenters could find them, even after virtually destroying the house. One priest hole, recently discovered by the owner of Mains Hall in Singleton, UK, was positioned behind plasterboard of a downstairs hallway, connected to a secret stairwell behind a bookshelf in the study.

Check out a video of a Priest Hole located at Naworth Castle. It was located behind a retractable wooden panel in the bottom of a door, which led to a subterranean stone enclosure.

http://www.medieval-castle.com/architecture_design/medieval_priest_hole.htm

More recently, secret passages and rooms have been used for nefarious purposes such as hiding illegal drugs, gambling operations, or alcoholic beverages, smuggling illegal immigrants or prisoners, or, even as torture chambers for deranged criminals. Other types of modern secret passages and rooms, such as Panic Rooms, are constructed by building companies as a refuge for families in case of a home invasion or burglary. These hidden rooms often feature soundproof and bulletproof walls and phone connections that enable inhabitants to call for help.