Source : Webster's Revised Unabridged Dictionary (1913)

Presidio \Pre*si"di*o\, n. [Sp.]
   A place of defense; a fortress; a garrison; a fortress; a
   garrison or guardhouse.

Source : WordNet®

presidio
     n : a fortress established in the southwestern United States by
         the Spanish in order to protect their missions and other
         holdings; "Tucson was first settled as a walled presidio"