Bailey - now used to mean the courtyard of any castle, but properly used refers to the space between a motte and its outer walls.
Barbican - the outer defensive works which protects the main entrance of a castle or town gates etc.
Bastion - fortified projections usually in the form of two flank walls which then turn to finish in a salient angle, usually built at the corners of fortifications, which allows the defenders a clear view of the ground below the curtains, enabling them to sweep it with fire.
Battlement - a fortified parapet in which the upstanding pieces are called merlons, while the indentations are the embrasures or crenels.
Caponier - a protected passageway with firing ports which extends into, or crosses a ditch, from which guns can be bought to bear along the length of the ditch.
Casemate - a vaulted chamber set into the walls usually with an embrasure. Sometimes used for firing guns they were more often used for storing amunition, garrison accommodation, stabling etc.
Citadel - a fort usually with bastions which is situated on the edge of a town and which forms part of the defences of the town. The Acropilis was the equivalent of the itadel in the Greek city.
Counterscarp - the exterior wall of a ditch, ie the side nearest the attackers.
Covered way - a path on the attackers side of the ditch sunk below the glacis and usually afforded protection by a parapet. Troops on the covered way were in effect, the first line of defence. Place of arms were spaces on the covered way, usually within a salient, where troops could muster.
Curtain - the main defensive wall of a fortification, usually split into sections, which are then also called curtains, by turrets or bastions.
Ditch - an excavation in front of a rampart which presents an obstacle to the attackers and provides excavated material which can be used in construction of ramparts. Commonly referred to as a "moat."
Embrasure - a small opening in a wall or parapet, usually with splayed reveals which allows guns to be fired from cover.
Enciente - the continuous outline or perimeter of a fortification, which is usually taken to follow the main line of defences.
Enfilade - fire from, for example, a bastion which is capable of raking along an advancing line of attackers, thereby inflicting maximum casualties.
Esplanade - cleared space on which people can walk, as a relief from the overcrowding of the town or fort, and which offers a field of fire to defenders, situated between a town and its citadel or immediately within the walls of a fortification.
Glacis - sloping ground on the attackers side of the covered way, cleared of all obstacles and therefore exposed to defensive fire.
Gorge - the rear face of an exterior defensive work.
Keep - the principal tower of a castle, usually in the centre. Donjon, not to be confused with dungeon, is the French term for keep.
Motte - a steep sided mound surmounted by a keep or other defendable structure which formed the central feature of early castles.
Ramparts - a thick wall behind the ditch, of earth, stone etc which is the main defensive wall of a fortification.
Salient - a line of defence which points towards the attackers in an arrow shape. The opposite of re-entrant.
Sally port - a discreet exit which allows the defenders to "sally forth" and engage the attackers.