After the introduction of the trébuchet, we might expect that the walls of keeps would be made very much thicker, and such seems to have been the case in France,[1193] but we do not find that it was the rule in England.[1194] The lower storeys were now generally instead of occasionally vaulted. In the course of the 13th century it became common to vault all the storeys. But in spite of the military advantages of the round keep, in its avoidance of angles favourable to the battering-ram, and its deflection of missiles, the square keep continued to be built in various parts of both France and England till quite late in the Middle Ages.[1195] On the Scottish border, square towers of the ancient type, with quite Norman decorations, were built as late as the 15th century.[1196] The advantage of the square tower was that it was more roomy inside, and was therefore preferred when the tower was intended for habitation. We come now to the greatest of all the changes introduced in the 13th century: the keepless castle, in which the keep is done away with altogether, and the castle consists of a square or oblong court surrounded by a strong wall with massive towers at the angles, and in large castles, in the curtain also.[1197] Usually this inner quadrangle is encircled with an outer quadrangle of walls and towers, so that this type of castle is frequently called the _concentric_. But the castles of the keepless kind are not invariably concentric; those built by Edward I. at Conway, Carnarvon, and Flint are not so.[1198] Instead of a dark and comfortless keep, the royal or noble owner is provided in this type of castle with a palatial house. In England this house is frequently attached to the gateway, forming what we may call a gatehouse palace; good examples may be seen at Beaumaris, Harlech, and Tonbridge.[1199] The gateway itself is always defended by a pair of massive towers. Edward I. is generally credited with the introduction of this type of castle into England, but until the _Pipe Rolls_ of Henry III.'s reign have been carefully examined, we cannot be certain that it was not introduced earlier. It was certainly known in Germany fifty years before Edward's accession to the throne, and in France as early as 1231.[1200] It is always supposed that this type of castle was introduced by the Crusaders from Syria. But when did it make its first appearance in Syria? This is a point which, we venture to think, has not been yet sufficiently investigated. We do not believe that it can have existed in Syria at the time of the third crusade, otherwise Richard I., who is universally acknowledged to have been a first-class military architect, would have brought the idea home with him.[1201] Yet his favourite castle of Chateau Gaillard, built in accordance with the latest military science, is in the main a castle of the keep-and-bailey type, and has even a reminiscence of the motte, in the scarped rock on which the keep and inner ward are placed.