That kind of architecture was, as was natural, especially in the South, to a considerable extent founded on that of the Romans. This will be more fully explained and illustrated, when we come to treat of the fortifications of Carcassonne, which are partly Roman or of Roman foundation. In the North the early fortresses consisted of earthen mounds, protected by palisades and ditches. Such were the defences of the native Gauls, which Cæsar found so boldly defended. To these succeeded the strong towers of masonry, of which the Norman keep is the well known type. Stone-built towers of that description gradually superseded the wooden fort set upon the top of an earthen “motte” or mound which formed the central stronghold of the earlier fortresses. Masonry then, step by step, took the place of wood in the defences; first, as we have seen, in the keep, and then in the enclosing walls. As the science of attack improved, the latter were made stronger, and were further fortified by the construction in connection with them of numerous strong towers. These were generally round in the North and square in the South. The means of active defence were chiefly from the parapet. At first the parapets of the walls and towers were armed in time of war with wooden enclosures, called “hoards” or “brétêches,” projected on short wooden beams. These enabled the defenders to overlook and protect the base of the works, which were then the weak points of the fortifications, and were liable to attack by sapping or mining. The assailants carried out this kind of assault by rolling up their sappers to the walls in “cats” or “sows” (small wagons strongly constructed and defended on the top with bags of wool and wet hides), which could only be destroyed by great stones and beams, hurled down from the projecting “hoards” above. The sockets for this wooden armature of the walls still exists unaltered in the thirteenth century fortifications of Carcassonne and Aigues Mortes. By degrees the wooden hoards were abandoned, being found liable to destruction by the fire balls or “Greek fire,” which the crusaders had learned the use of in the East. Parapets of masonry were then substituted for them, projected on bold stone corbels, which left intervals between the parapet and the face of the wall, called” machicolations,” through which the defenders could rain missiles on the assailants. In the fourteenth century these corbelled parapets are amongst the most prominent and picturesque features of the castles and fortifications of the period. In course of time the stone parapets were further improved and heightened into several storys, the lower ones being covered, and the upper forming an open crenellated walk. In the fifteenth century this system reached its height, and produced in the lofty towers and walls, crowned with their numerous boldly overhanging works, some of the most magnificent works of the military architecture of the Middle Ages.