acropolis advance guard airhead avant-garde bastion battle line blockhouse bridgehead bunker castle citadel donjon farthest outpost fasthold fastness first line forefront fort fortress front front line front rank front-runner garrison garrison house hold keep line martello martello tower mote motte outguard outpost peel peel tower pillbox pioneer point post precursor railhead rath safehold scout spearhead strong point stronghold tower tower of strength van vanguard ward