guardroom

from WordNet (r) 3.0 (2006)

guardroom
    n 1: a cell in which soldiers who are prisoners are confined
    2: a room used by soldiers on guard

from The Collaborative International Dictionary of English v.0.48

Guardroom \Guard"room`\, n. (Mil.)
   The room occupied by the guard during its term of duty; also,
   a room where prisoners are confined.
   [1913 Webster]