A Glossary of Castle Terminology

Castles

Arbalestina: Cruciform loophole, used by crossbowmen.
Barbican: Advanced fortification work protecting the gateway of a city or castle.
Bartizan: Projecting corner turret

These are particularly notable in the castles of North Wales
Bastion: Fortification work, usually four-sided (tetrahedral) and situated in the corners of a wall, with a sharp point, making possible active defence and flanking fire.
Bastion Tower: Tower projecting from the wall face and functioning as a bastion.
Batter: Outward slope of a revetment.
Battery: Grouping of artillery.
Brattice: Temporary wooden gallery for use in a siege.
Caponnière: Covered passage across a ditch to an outer fortification structure such as a ravelin
Casemate: Covered chamber for musketry or artillery.
Cavalier: Raised structure containing a battery, usually sited above the centre of a bastion. Raising the battery gives a better trajectory.
Citadel: Heavily fortified, independent defensive structure within city walls, dominating an ancient or medieval town; in the bastion system, the strongest part of the fort.
Corbel: Supporting bracket projecting from the face of a wall.
Counterguard: Detached structure protecting a bastion.
Counterscarp: Outer face of a ditch
Couvre Face: Low rampart in the ditch, protecting the face of a ravelin; analogous in function to the counterguard and fausse braye.
Covered Way: Passage along the outer edge of a ditch, protected by a rampart forming the glacis; it is usually some 10 metres wide.
Crownwork: Advanced fortification structure screening the entrance to a bastion fort.
Curtain, Curtain Wall: Stretch of wall betwen two towers.
Donjon: Large main building of a medieval castle. Keep is an alternative name for donjon, particularly widely used down to the 12th century.
Fausse Braye: Low rampart in the ditch, in front of the main fortification.
Glacis: Rampart of the Covered Way, sloping away towards the enemy.
Featured in the song "The British Grenadiers"
Hornwork: 'Horned' structure, consisting of two demibastions, one at each end of a curtain.
Keep: See Donjon
Lunette: Triangular (originally crescent-shaped) defensive structure, open at the rear, often flanking a ravelin
Machicolation: Masonry gallery supported by corbels and provided with openings through which missiles could be dropped onto attackers below.
Crenel: The gap in crenellations.
see Merlon
Crenellations: A "toothed" upper edge to a parapet wall.

see Crenel & Merlon
Merlon: The "tooth" in crenellations, alternating with gaps.
see Crenel
Motte: Artificial mound.
Neck Ditch: Ditch cutting across a neck of land, to hinder an enemy's advance.
Bailey
Orillons: Round or squared-off extensions of a bastion's faces, designed to protect its recessed flanks.
Piatta Forma: Fortification structure protecting the curtain between two bastions; it is square or rectangular in plan, or takes the form of a small tetrahedral bastion.
Pilaster: Vertical strip slightly projecting from the wall face.
Place of Arms: Enlarged area in a covered way, where troops could assemble.
Postern: Small gateway in a castle from which the besieged could sally forth.
Ravelin: Fortified structure, more or less triangular in plan, sited in the ditch to defend the curtain.
Redoubt: Detached, independent outwork.
Reduit: Detached, independent outwork.
Retrenchment: Interior defence works, usually consisting of a trench and parapet.
Revetment: Retaining wall; i.e. a wall supporting a bank of earth or mass of water
Roundel: Low, circular, semicircular or U-shaped tower for artillery, projecting from the wall face.
Rustication: Masonry made of rough stone whose outer surface is only roughly dresssed. Also known as en bosse
Scarp: Inner face of a ditch.
Shell Keep: Castle consisting of a high strong ring wall with residential buildings on its inner face.
Examples: Clifford's Tower - York, Arundel Castle
Shield Wall: Exceptionally thick wall, protecting the castle on its most vulnerable side.
Tenaille: Low earth or masonry structure, built in the ditch to protect the curtain.
Trace: Circuit or fortified perimeter, also known as the enceinte.
Traverse: Small bank or wall that cuts across the line of a covered way.

Siege Engines

Catapult
Perrier: A stone throwing engine, powered by teams of soldiers pulling on ropes
Ballista
Trebuchet: A heavy stone throwing engine, powered by a large counterweight
Onager: Roman name for a Mangonel, literally a "wild ass"
Mangonel: A light stone throwing engine, powered by a skein of twisted rope.