stronghold (n.)

• acropolis
• asylum
• bastion
• beachhead
• blockhouse
• bridgehead
• bulwark
• bunker
• castle
• citadel
• donjon
• fastness
• fort
• fortification
• fortress
• garrison
• harbor
• harborage
• haven
• hold
• keep
• mote
• peel
• pillbox
• port
• post
• preserve
• rath
• redoubt
• refuge
• safehold
• sanctuary
• tower
• ward