Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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