The strength of a mediæval fortress lay in the extraordinary solidity of its construction. Against walls fifteen to thirty feet thick, the feeble siege-artillery of the day, perriéres, catapults, trebuchets, and so forth, beat without perceptible effect. A Norman keep, solid and tall, with no wood-work to be set on fire, and no openings near the ground to be battered in, had an almost endless capacity for passive resistance. Even a weak garrison could hold out as long as its provisions lasted. Mining was perhaps the device which had most hope of success against such a stronghold[66]; but if the castle was provided with a deep moat, or was built directly on a rock, mining was of no avail. There remained the laborious expedient of demolishing the lower parts of the walls by approaches made under cover of a pent-house, or ‘cat,’ as it was called. If the moat could be filled, and the cat brought close to the foot of the fortifications, this method might be of some use against a fortress of the simple Norman type. Before bastions were invented, there was no means by which the missiles of the besieged could adequately command the ground immediately below the ramparts. If the defenders showed themselves over the walls--as would be necessary in order to reach men perpendicularly below them--they were at once exposed to the archers and cross-bowmen who, under cover of mantlets, protected the working of the besieger’s pioneers. Hence something might be done by the method of demolishing the lower parts of the walls: but the process was always slow, laborious, and exceedingly costly in the matter of human lives. Unless pressed for time a good commander would almost invariably prefer to starve out a garrison.