a strong or fortified place occupied by troops and usually surrounded by walls, ditches, and other defensive works; a fortress; fortification.

any permanent army post.

(formerly) a trading post. ?

hold the fort

to defend one's position against attack or criticism.

to maintain the existing state of affairs.

noun

acropolis , blockhouse , camp , castle , citadel , fastness , fortification , fortress , garrison , redoubt , station , base , bastille , bastion , bulwark , fortify , protection , stronghold