Castles and Siegecraft
The king of France [ Philip II] turned his attention to the construction of machines and rocklhrowers, suitable for attacks and which he determined to ply night and day, and he had one of superior quality, to which they gave the name of "Bad Neighbor." The Turks also had one they called "Bad Kinsman," which by its violent casts, often broke "Bad Neighbor" in pieces; but the king rebuilt it, until by constant blows, he broke down part of the principal city wall. There was one rockthrower, erected at common expense, which they called the "petraria of God." Near it, there constantly preached a priest, who collected money to restore it and to hire persons to bring stones for casting. By means of this engine, a part of the tower Maledictum was at length shaken dawn. In addition to these two, King Richard had constructed two others of choice workmanship, which would strike a place at an incalculable distance. These engines were plied day and night, and it is well known that a stone sent from one of them killed twelve men with its blow. Such stones and flinty pieces of rock, nothing could withstand; but they either shattered into pieces the object they struck or ground it to powder.
Geoffrey de Vinsauf, "Chronicle of Richard the Firsts Crusade", in Chronicles of the Crusades ( London, 1848; reprint New York, 1969), pp. 205-06. Vinsauf was describing the siege of Acre in 1191 during the Third Crusade.
By the fourth century A.D., the level of Roman expertise in fort building had declined significantly. The technical skills required for building quality forts had essentially disappeared from western Europe by 500. The highly durable character of numerous Roman fortifications did permit many of them to be used well into the Middle Ages, and to serve as models long after.
The disappearance of ancient skills in masonry and fort design meant that timber and earth piled in simple structures were the materials for most fortifications of the early Middle Ages, although old Roman walls were occasionally kept in good repair or quarried for stone and brick. For the