bridgehead (n.)

• acropolis
• airhead
• avant-garde
• bastion
• beachhead
• blockhouse
• bunker
• castle
• citadel
• donjon
• fastness
• forefront
• fort
• fortress
• front
• front-runner
• garrison
• hold
• keep
• line
• mote
• outpost
• peel
• pillbox
• pioneer
• point
• post
• precursor
• railhead
• rath
• safehold
• scout
• spearhead
• stronghold
• tower
• van
• vanguard
• ward