From Clark's Medieval Military Architecture, by permission of Bernard Quaritch
Fig. 1.—Plan of Laughton en-le-Morthen. its summit was placed a timber palisade. This moated mound was styled in French motte (latinized mota), a word still common in French place-names. It is clearly depicted at the time of the Conquest in the Bayeux tapestry, and was then familiar on the mainland of western Europe. A description of this earlier castle is given in the life of John, bishop of Terouanne (Acta Sanctorum, quoted by G. T. Clark, Medieval Mil. Architecture):— "The rich and the noble of that region being much given to feuds and bloodshed, fortify themselves ... and by these strongholds subdue their equals and oppress their inferiors. They heap up a mound as high as they are able, and dig round it as broad a ditch as they can ... Round the summit of the mound they construct a palisade of timber to act as a wall ... Inside the palisade they erect a house, or rather a citadel, which looks down on the whole neighbourhood." St John, bishop of Terouanne, died in 1130, and this castle of Merchem, built by "a lord of the town many years before" may be taken as typical of the practice of the 11th century. But in addition to the mound, the citadel of the fortress, there was usually appended to it a bailey or base court (and sometimes two) of semilunar or horseshoe shape, so that the mound stood à cheval on the line of the enceinte. The rapidity and ease with which it was possible to construct castles of this type made them characteristic of the Conquest period in England and of the Anglo-Norman settlements in Wales, Ireland and the Scottish lowlands. In later days a stone wall replaced the timber palisade and produced what is known as the shell-keep, the type met with in the extant castles of Berkeley, Alnwick and Windsor.
But the Normans introduced also two other types of castle. The one was adopted where they found a natural rock stronghold which only needed adaptation, as at Clifford, Ludlow, the Peak and Exeter, to produce a citadel; the other was a type wholly distinct, the high rectangular tower of masonry, of which the Tower of London is the best-known example, though that of Colchester was probably constructed in the 11th century also. But the latter type belongs rather to the more settled conditions of the 12th century when haste was not a necessity, and in the first half of which the line extant keeps of Hedingham and Rochester were erected. These towers were originally surrounded by palisades, usually on earthen ramparts, which were replaced later by stone walls. The whole fortress thus formed was styled a castle, but sometimes more precisely "tower and castle," the former being the citadel, and the latter the walled enclosure, which preserved more strictly the meaning of the Roman castellum.
Reliance was placed by the engineers of that time simply and solely on the inherent strength of the structure, the walls of which defied the battering-ram, and could only be undermined at the cost of much time and labour, while the narrow apertures were constructed to exclude arrows or flaming brands.
At this stage the crusades, and the consequent opportunities afforded to western engineers of studying the solid fortresses of the Byzantine empire, revolutionized the art of castle-building, which henceforward follows recognized principles. Many castles were built in the Holy Land by the crusaders of the 12th century, and it has been shown (Oman, Art of War: the Middle Ages, p. 529) 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.
Fig. 2.—Vertical section of rectangular Norman Keep (Tower of London)
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.
But the finest of all castles of the middle ages was Richard Cœur de Lion's fortress of Château Gaillard (1197) on the Seine near Les Andelys. Here the innermost ward was protected by an elaborate system of strong appended defences which included a strong téte-de-pont covering the Seine bridge (see Clark, i. 384, and Oman, p. 533). 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 á outrance, and it