pre·si·di·o 
(prĭ-sēdē-ō′, -sĭdē-ō′)
(prĭ-sēdē-ō′, -sĭdē-ō′)n. pl. pre·si·di·os
A garrison, especially a fortress of the kind established in the southwest United States by the Spanish to protect their holdings and missions.
[Spanish, from Latin praesidium, guard, defense, from praesidēre, to guard; see PRESIDE.]