The Collaborative International Dictionary of English v.0.48:

Guardhouse \Guard"house`\, n. (Mil.)
   A building which is occupied by the guard, and in which
   soldiers are confined for misconduct; hence, a lock-up.
   [1913 Webster]

WordNet (r) 3.0 (2006):

guardhouse
    n 1: a military facility that serves as the headquarters for
         military police and in which military prisoners can be
         detained