Trébuchet with ropes attached to counterpoise
This description suggests that the beam, balanced on an axis, and needing several people to attend to the discharge at one end, was worked by a counterpoise at the other. This was the case with the developed slinging machine, known as trébuchet. A pole, working on a pivot between two upright stands, was weighted at one end with a heavy wooden chest, filled with earth, which kept the pole, when not in use, in a vertical position. To the other end was attached a long sling, capable of containing large stones. When the tension of the ropes which dragged the pole backwards and lifted the counterpoise was released, the counterpoise fell heavily, bringing the pole abruptly back into position, and the sling, describing a circle in the air, let fly the stone when it reached the summit of the arc ([75]). Variations of this form of catapult, which became general in the thirteenth century, are found ([76]); the machine known as cabulus which Philip Augustus used with excellent effect against the strong inner wall of Château-Gaillard, was possibly worked upon the principle of counterpoise.
Against these modes and machines of attack the defenders of a castle had to contend. The obvious means of defence was to oppose to the enemy a thickness of wall which would be proof against the blows of the ram or the slow labour of the pick. But even the very strong inner wall at Château-Gaillard, which was constructed with the special object of resisting these engines, yielded to the miners, reinforced by the great slinging machine. In this instance the castle had undergone a long blockade; its communications had been cut off some months beforehand; and the garrison was greatly reduced in numbers. The lesson of the siege was that against a persistent and well-conducted blockade mere passive strength was of little avail. Here, too, the defenders, driven back from one bailey to another, seem to have renounced the opportunity of final shelter afforded them by the keep, and to have made an attempt to evacuate the castle by a postern before they fell into the hands of the enemy.
Aigues-Mortes
Carcassonne
Château-Gaillard, however, and the castles of its period will be discussed in detail in the sequel. At present, we are concerned with the direct methods employed to meet the attack of siege-engines and attempts at escalade. Against the great catapults the besieged were practically powerless. The use of such machines upon the walls themselves was as dangerous to the stability of the masonry as their use by the enemy, and hastened the chance of a breach: they could not be employed from the interior of the enclosure, without endangering the defenders on the rampart.[94] The summit of the rectangular keep of the twelfth century was never constructed as a platform for artillery: here, again, engineers probably feared the effect of the constant vibration upon a flat wooden roof, and were content to conceal their ridged roofs within high ramparts. The main arm of defence which could be employed by the defenders was the cross-bow. Their superior position upon the ramparts enabled them to throw down stones and burning material upon the assailants engaged at the foot of the wall, and the wheeled belfries formed a direct target for their arrows. The ram could also be paralysed by letting down grappling-irons or beams with forked heads, which gripped and disabled it; or sacks of wool or earth could be lowered to meet its strokes. The assailants, however, worked under their defences of pent-houses and mantlets, the solid tops and sloping surfaces of which were specially devised against the shock of stones and arrows; while, as we have seen, their coverings were so protected that it was difficult for them to catch fire.
Parapet defended by hoarding, showing elevation, section through hourd and coursière, and method of construction