citadel, bastion - a stronghold into which people could go for shelter during a battle

blockhouse - a stronghold that is reinforced for protection from enemy fire; with apertures for defensive fire

defensive structure, defense, defence - a structure used for defense; "the artillery battered down the defenses"

hold - a stronghold

donjon, dungeon, keep - the main tower within the walls of a medieval castle or fortress

redoubt, sconce - a forbidding stronghold