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

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "guardhouse":
   POW camp, bastille, black hole, borstal, borstal institution,
   bridewell, brig, cell, concentration camp, condemned cell,
   death cell, death house, death row, detention camp, federal prison,
   forced-labor camp, gaol, house of correction, house of detention,
   industrial school, internment camp, jail, jailhouse, keep,
   labor camp, lockup, maximum-security prison,
   minimum-security prison, oubliette, pen, penal colony,
   penal institution, penal settlement, penitentiary, prison,
   prison camp, prisonhouse, reform school, reformatory,
   sponging house, state prison, stockade, the hole, tollbooth,
   training school