In the Middle East the Crusaders developed great castles with double circuits of curving outer walls and towers or turrets to overlook all sections of the wall. The form of these castles had an influence throughout the Continent and the British Isles. Thus early in the 13th cent. the medieval castle, a mixture of Norman, English, and Byzantine elements, reached its full flower, as typified in the Château Gaillard on the Seine in France and in Alnwick and the Conisborough in England.
In general, the castle was planned for security; the living quarters were rude, poorly lighted, and without provisions for comfort. Typically, the keep contained the living quarters of the lord and his family, the rooms of state, and the prison cells. Two independent systems of walls, each a fortress in itself, extended around the keep; the sections of the walls were flanked by towers, usually round, and the principal entrance was protected by strong gate towers, the massive gateway, with its portcullis and drawbridge, and the barbican, or advanced outwork. The defenders operated from galleries at the tops of walls and from the flat roofs of towers, whose battlements were provided with recesses with flaring sides, called embrasures, and openings, or machicolations, for shooting and dropping missiles on the attackers. The fully developed castle was thus marked by successive series of defenses; the fall of the outer works did not necessarily mean the loss of the entire castle.
With the use of gunpowder and consequent perfection of artillery, the castle lost its military importance. The manor house replaced the castle as the residence of the wealthy landowner, but the architectural influence of the castle has persisted even to the present day, when crenelations and towers are still found in country houses and some urban structures.