At Constantinople the crusading hosts before the city found themselves confronted by a comparatively low fortified wall, bristling with impediments; within it, at the distance of some hundreds of feet, arose another and taller wall, while beyond that again a third wall, the highest of all, appeared. These walls extended for more than three miles upon the western side, with one hundred towers; all were embattled, and they offered a stupendous scene to the wondering eyes of the Crusaders as they vanished in grand perspective into the distance. There is no castle in England which presents more than three hundred yards of continual front. The capture of the first defence of the eastern capital by no means imperilled the integrity of the second, while the prospective losses of the assailants when confined in the narrow space between the first and second lines was appalling to con template. The same difficulty would occur with regard to the second and third lines of defence, and it is small wonder that the leaders paused in a projected attack upon so formidable an obstacle. [Illustration: CASTLE RUSHEN, ISLE OF MAN.] The essential principles underlying the construction of a castle erected upon the concentric plan were:-- 1. That the natural features of the selected site should be adapted and made part of the defences, and that no rigid plan of the ground occupied, based upon former principles of castellation, should be strictly followed. 2. That a series of defences independent and complete in themselves should be presented in turn to an assault, the capture of one by no means entailing that of another. The castle-builders of the second half of the thirteenth century rigidly adhered to the principles embodied in the first clause given above; they did not produce a structure of the Motte and Bailey, or the Keep and Base-court types, with little regard to the situation and configuration of the ground, but made their plans with the utmost care, embracing every advantage which the site presented. As a necessary sequence the ground plan of one Concentric Castle differs from every other, and it is only by a general summary of the ideas prevailing that any comparison can be made. The second clause naturally suggested a concentric plan whereby each defence was placed within the other, the strongest of all naturally being in the centre. But as most of the English castles were rendered concentric by means of additions to buildings previously existing, the pure concentric ideal is seldom reached except in those structures reared entirely at that period, the others attained it more or less by developing conditions already obtaining. [Illustration: THE IDEAL CONCENTRIC CASTLE.]