Siege engines were devices designed to reduce the time taken to capture a besieged castle or other fortification. There were a number of different types, the most prominent and certainly the simplest to construct being the battering ram. In its primitive form this was little more than a large beam or tree trunk that could be propelled continually against a wall or gate until a breach was made. It was frequently capped with metal (sometimes a symbolic ram's head) to strengthen the striking surface, and the survival of those operating it was greatly enhanced when it was covered by an armoured roof structure, from which it could also be suspended to improve impact frequency and accuracy. The digging machine was developed to aid sappers to undermine walls and usually consisted of a rotating beam equipped with an iron head which could drill through earth, stone, and mortar, again under an armoured roof. The latter developed into a siege engine in its own right, with wheels or rollers and with sloping sides covered with leather or metal.
Torsion and counterpoise engines of war were widely used by the Greeks and Romans and throughout the Middle Ages by Europeans, Chinese, and Arabs and, following their example, the Mongols. Although smaller engines like the ballista were sometimes used in the field their principal function was to attack fortifications. Trebuchets could throw the biggest projectiles, although the most powerful catapults achieved greater range, velocity, and accuracy
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But the most technologically complicated siege engine was the siege tower or beffroy, built to be taller than the walls of the work under attack and consisting of an elevated platform from which archers could bring the ramparts under fire and ramps lowered to permit an assault. The bigger siege towers contained internal stairs so that once in place they could pour a continual flow of assaulting soldiers onto the ramparts, obviating the need for a breach. These too would be made of wood and covered with leather, water-soaked skins, or wicker to protect them against projectiles and fire, and were advanced on rollers or their own wheels.
Before the advent of gunpowder, siege artillery went by the generic name of catapults and worked on torsion, traction, or counterweight. The earliest were torsion catapults, derived in the 4th century bc Syracusan gastraphetes or ‘belly-bow’. The gastraphetes was a large, powerful, and flexible crossbow mounted on a heavy stock. Although it could propel a projectile further than the composite bow, it was still too weak to breach walls or gates. To gain this needed power, ancient engineers constructed torsion springs made of tightly twisted sinew to which they anchored two rigid bow arms. Mounted onto a stock with the addition of a winch, ratchet apparatus, and trigger mechanism, much further distances and much greater ballistic forces were achieved. Philip II of Macedon and Alexander ‘the Great’ used them to capture many towns in Greece and the Middle East. The Romans and Carthaginians continued to use torsion artillery, making some improvements in the housing of the springs and in the stock. The Romans also made them smaller and portable. Using the same torsion technology, they also created the onager, the familiar single-spring, one-armed catapult used to launch larger projectiles.
Torsion catapults continued to be built into the time of the barbarian invasions when they were superseded by a traction artillery piece, the trebuchet. Diffused westward from China, where they were invented some time during the 5th to 3rd centuries bc, trebuchets were designed with a rotating beam placed unevenly on a fulcrum which was supported by a wooden tower and base. On the longest side of the rotating beam was hung a sling in which projectiles would be placed; on the opposite side were hung between 40 and 125 ropes which, when pulled in unison, would propel a projectile with great ballistic force. These were used from the early to the high Middle Ages. It was also the principal siege weapon used by both Christians and Muslims during the Crusades. Toward the end of this period, in the mid-12th century, the trebuchet also underwent a significant change by the substitution of the pulling ropes with a fixed counterweight. This counterweight, which was little more than a box filled with stones, sand, or other heavy material, provided the power to discharge any large missile. By the mid-13th century all trebuchets had counterweights.
In the early 14th century, gunpowder artillery developed in Europe and Asia and by the end of that century they were in use in siege warfare. Initially, these were very large weapons made of iron bars held together by iron rings. They were immensely heavy, difficult to operate, and slow to fire, and their size diminished as greater muzzle velocity permitted smaller projectiles to produce the same effect. By the 17th century all such weapons were made in cast iron or bronze. Gunpowder was also employed in sieges in the form of petards placed against a gate or other obstacle. The last use of siege engines in a major engagement was in April 1945 when US forces used a siege tower on a specially adapted landing craft to assault Fort Drum, the ‘concrete battleship’ in Manila harbour. They disabled the gun cupolas with satchel charges (the descendants of the petard), poured 10, 000 gallons of gasoline into the ventilation shafts, lit fuses, and hastily retired to a safe distance. It burned for a week, amid secondary explosions that completely wrecked it.
Bibliography
- Bradbury, Jim, The Medieval Siege (Woodbridge, 1992).
- De Vries, Kelly, Medieval Military Technology (Peterborough, 1992).
- Hogg, Ian, Fortress (London, 1975).
- Landels, J. G., Engineering in the Ancient World (Berkeley, 1978)
— Kelly DeVries
