hidden passageway
A passage or stairway inside or between buildings which is architecturally concealed.
A hidden or secret passageway is a conduit for persons to connect between rooms, floors, or different buildings, which is architecturally concealed so as to prevent discovery. These features are built occasionally in castles, palaces, and mansions, as well as rare cases of institutional buildings. Some of these have been made public when a building opens as a museum; however this publicity does not negate their status as hidden passageways.
Concealment is part of the architecture of the route. A connecting stair or passage can qualify whether it joins rooms within a building or connects separate buildings.
Restricted access alone does not make an ordinary corridor a hidden passageway. Conversely, a concealed route remains this kind of feature when its existence becomes publicly known or it is shown to museum visitors.
