Many castles were built in the Holy Land by the crusaders of the 12th century, and it has been shown that the designers realized, first, that a second line of defences should be built within the main enceinte, and a third line or keep inside the second line; and secondly, that a wall must be flanked by projecting towers. From the Byzantine engineers, through the crusaders, we derive, therefore, the cardinal principle of the mutual defence of all the parts of a fortress.
The donjon of western Europe was regarded as the fortress, the outer walls as accessory defences; in the East each envelope was a fortress in itself, and the keep became merely the last refuge of the garrison, used only when all else had been captured. Indeed the keep, in several crusader castles, is no more than a tower, larger than the rest, built into the enceinte and serving with the rest for its flanking defence, while the fortress was made strongest on the most exposed front. The idea of the flanking towers (which were of a type very different from the slight projections of the shell-keep and rectangular tower) soon penetrated to Europe, and Alnwick Castle (1140-1150) shows the influence of the new system.
In Richard Cœur de Lion's fortress of Château-Gaillard Les Andelys, the innermost ward was protected by an elaborate system of strong appended defences, which included a strong fte-de-pont covering the Seine bridge. The castle stood upon high ground and consisted of three distinct enceintes or wards besides the keep, which was in this case merely a strong tower forming part of the innermost ward. The donjon was rarely defended until the very end and it gradually lost in importance as the outer "wards" grew stronger. Frederick II's Castel del Monte in Puglia has no keep at all: rising on a strategic alture, it consist of an octagonal structure with eighth polygonal, massive towers.
Round instead of rectangular towers were now becoming usual, the finest examples of their employment as keeps being at Conisborough in England and at Falaise and Coucy in France. Against the relatively feeble siege artillery of the 13th century a well built fortress was almost proof, but the mines and the battering ram of the attack were more formidable, and it was realized that corners in the stonework of the fortress were more vulnerable than a uniform curved surface. Château Gaillard fell to Philip Augustus in 1204 after a strenuous defence, and the success of the assailants was largely due to the wise and skilful employment of mines. An angle of the noble keep of Rochester was undermined and brought down by King John of England in 1215.
The next development was the extension of the principle of successive lines of defence to form what is called the "concentric" castle, in which each ward was placed wholly within another which enveloped it; places thus built on a flat site (e.g. Caerphilly Castle) became for the first time more formidable than strongholds perched upon rocks and hills such as Château Gaillard, where the more exposed parts indeed possessed many successive lines of defence, but at other points, for want of room, it was impossible to build more than one or, at most, two walls. In these cases, the fall of the inner ward by surprise, escalade, vive force, or even by ordinary siege (as was sometimes feasible), entailed the fall of the whole castle. The adoption of the concentric system precluded any such mischance, and thus, even though siege engines improved during the 13th and 14th centuries, the defence, by the massive strength of the concentric castle in some cases, by natural inaccessibility of position in others, maintained itself superior to the attack during the latter Middle Ages.
Construction of castles in this period was often connected to the necessity to establish a strong central power against local fragmentation, or in newly conquered lands: examples are the large buildings programs of Edward I of England in Wales, Philip I August of France, the Ezzelino IV da Romano and the Scaligers in northern Italy, Frederick II and Charles I of Anjou in southern Italy (often reusing former Norman or even Byzantine and Lombard structures), King Denis I in Portugal, and notably the Teutonic Knights in their conquest of Pagan lands in Prussia and Poland. In Germany, stone structures appeared in Hesse, Thuringia, Alsace and Saxony, commissioned by the powerful local aristocracy. Structures in northern Germany were usually simpler, often taking advantage of water streams.
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