Battlement: A parapet, with crenelation, atop a fortress wall, from which defenders could fire down on attacking troops.
Bent entrance: An entrance passage into a castle that makes an abrupt turn. The turn inhibited the use of a battering ram, confused and slowed down the attackers, and might make them vulnerable to missiles or fluids from machicolations over head. The long entry passage in the Krac des Chevaliers had three such turns.
Communications: Castles were often built on ridge lines within sight of each other so that signal fires could be used at night to pass on an alarm. Both the Arab armies and the Crusaders also used carrier pigeons.
Concentric Fortifications: Whenever the site permitted, castles had two lines of defense, the inner wall on higher ground being taller than the outer wall. Towers and loopholes were arranged so that the inner wall was never directly behind or above similar features in the outer wall, doubling their effect.
Crenelation: In battlements, which alternate open and solid spaces, the notches are crenels and the solid intervals merlons. Merlons were sometimes furnished as loopholes.
Keep: Usually the highest innermost tower, often built to overlook and thus strengthen the most vulnerable sectors of the castle's defenses. It was also the point from which the commander might direct the defense.
Loopholes (Arrow slits): In later castles, walls and towers were pierced at every level by loopholes or slits through which arrows or other missiles could be fired. The slits were often widened at the bottom in to a stirrup shape to broaden their fields of fire, and so that none was directly above another.
Machicolations: An opening between the corbel stones of a protecting roof through which missiles or hot fluids could be discharged on assailants below.
Moat: a deep wide trench, sometimes filled with water, that served as a barrier around a fortified castle.
Parapet: A low wall or breastwork that protected the edge of a platform or the walk along the top of the large wall.
Postern: A small gate at the side or back of a castle, usually in a concealed spot, such as a recess in the angle of a square tower. The postern, also called sally port, permitted small offensive sorties and allowed messengers to come and go inconspicuously.
Portcullis: A heavy grating hung above a fortified gateway; it could be lowered, sliding in stone groves, to block the entry.
Talus: The much thickened lower portion of a castle's curtain wall, designed to prevent attackers from getting too close to the base of the wall, or directly beneath the towers, and battlements, where they might be hidden from the line of fire. The talus gives the lower half or third of the walls a distinct outward slope.
Towers: Set at intervals in both outer and inner walls, towers were strong points from which fire could be concentrated. In early fortresses they were placed at corners or turns in the wall; later structures had more towers placed more frequently and protruding from the castle walls to permit flanking fire along them. Square towers gave way to round ones, again improving the field of fire and making the tower less susceptible to artillery fire or undermining.