A tower at the entrance to a castle or fortified town
An opening in the wall of a fortress through which the guns are levelled; a narrow loophole through which arrows and other missiles may be shot.
A fortress at the end of a bridge.
A temporary wooden tower built for defensive purposes.
The act or process of making a building into a castle.
The addition of battlements to a building, as for example when a feudal manor house needed fortification against border-raider attacks.
Any of a set of slots or grooves in an object, often machined into a metal part, as for example on a castellated nut.