(n.) A building of masonry, generally circular, usually erected on the seacoast, with a gun on the summit mounted on a traversing platform, so as to be fired in any direction.

bastion beachhead blockhouse bridgehead bunker castle citadel donjon fasthold fastness fort fortress garrison garrison house hold keep martello martello tower mote motte peel peel tower pillbox post rath safehold strong point stronghold tower tower of strength ward