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- the whole purpose of fortification is to enable smaller numbers to resist more powerful attacking forces. The three basic elements when defending a site were walls, towers and gates. Defence is a natural reaction and most people are able to identify personally with defensive structures amongst which can be included many of the worlds best known buildings. Initially, impregnable sites were chosen and were usually developed on the basis of strong buildings within a number of concentric rings of defences, which ideally, would rise higher and higher so that those in the outer defences could be given some protection by those in higher positions behind them. Usually, there would only be one entrance to such a complex and it was difficult for those within to embark on any positive action. T.E. Lawrence famously pointed out that any entrance that could be defended by one man could almost certainly be besieged by two, standing on each side of the doorway thereby preventing anyone getting out. The invention of gunpowder and the subsequent advent of cannon fire called for radical rethinks in fortification, not only had walls to be stronger, but defenders had to be given the ability to retaliate against any attackers, otherwise their positions would simply be pounded to rubble. This problem exercised many of the finest minds from the early Renaissance and made military engineering one of the most important and prestigious of professions. The French were usually in the forefront, and probably the most famous military engineer was Sebastian Vauban (1633-1707), many of whose fortresses were in military use up until the 1914 -18 war.

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.