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.
The ideal concentric outline may be gleaned from the accompanying plan, where the three entrances are a special feature, each being placed as far as possible from the one adjacent. By this device the assailants who had managed to capture the outer enceinte would be compelled to pass under one half of the second line of towers and curtain walls before reaching the entrance pierced through them, being all the time subjected to a plunging fire of deadly missiles. The same would occur if the second line were captured. The gates were in all cases flanked by defensive towers, and generally reached by a drawbridge which could be raised before the entrance archway; this was narrow and defended by one or more portcullises, while a strong gate, usually sheathed with iron, was placed at the entrance immediately behind the raised drawbridge. If these formidable obstacles were overcome and the first part of the passage captured the inner portcullis or portcullises had to be forced, but the assailants would in the meantime be subjected to a galling discharge of arrows and bolts from the narrow loopholes on either side, which were pierced in the walls of rooms whose only entrances were from the inner courtyard or from the ramparts. In the vaulted roof of the passage also circular openings were built, termed "meurtriers," or murderers, through which melted lead, hot water or oil, and other liquids could be poured upon the struggling mass of assailants below. From the formidable nature of the defence it may readily be understood that direct assaults of castles built upon the concentric ideal were limited, the besiegers contenting themselves with waiting until famine had done its work, or treachery within the walls allowed them to enter. The project of capturing three strong castles, one within the other, was a prospect sufficient to daunt any ordinary commander, and so long as the besieged could count upon a friendly army in the field outside, the loyalty of the garrison, and a plentiful supply of provisions, the fortress might be relied upon to maintain its integrity.
