Essentially there were two forrm of combat in Medieval Europe, the skirmish and the siege. It was rare that two armies would take to the field in hopes of open combat on a field of battle. The limiting factors that decided this was lack of funds and resources to equip and maintain a standing army. Plunder was the dominant enticement for volunteers. Living off conquered territories was the preferred method to upkeep the supply base for an invading force.
In order to gain control of an area and allocate its resources, an invading force would have to defeat the garrison forces in the local citadel or fortress. That required a siege. The idea behind a siege was to terrorize the surrounding populace and drive them to the local lord's fortress, for a couple of reasons. First, in preparation of a siege it was necessary to limit or cut off the resources entering the castle: relief forces, foodstuffs, water, ect. Second, increasing the population in a fortress while at the same time cutting off the resources available to the castle shortened the length of the siege. An increased population need more food and at the same tome improves the likelyhood of disease and plague.
After the fortress was barred and lie in wait of the enemy there were four main threats that an attacking army employed to gain control of the castle and consequently the surrounding resource base. The strongest weapon and most common reason for surrender of a fortress was starvation and disease. After an extended siege, there were no pets left, the rat population was down, and occasionally there were reports of cannibalism. In order to shorten the siege, the army camped outside the fortress would try to gain access to the castle through three means: mining the walls, construction of siege towers often with battering rams as their main component, and bringing forth the siege engines. The towers were mainly used to destroy the main gates or gain access to an elevation allowing the assulting force to gain the level of the walls to invade.
Another way to invade a citadel was to go to work on the walls and create a breach for access. Often by the time a breach occured the remaining force within was so weakened by disease and starvation defense was futile. To create a breach in the walls, the Medieval commander had two methods of attack at his disposal. Mining beneath the walls was done to collapse a section of wall and ultimately was the most reliable mothod of entry. Another way to gain access was to create a breach using siege engines, or heavy mechanical artillery.
Siege Engines, or heavy mechanical artillery were used from the times of antiquity up until the advent and dominance of reliable cannon. These machines were referred to as 'gyns' and the men in control of these machines were called 'gyneers'. These machines were extremely valuable and large. The gyneer was usually the highest paid man on the field, in part due to the high education and experience needed to maintain and fire these machines. Incredible expense and effort was under taken to transport and assemble these machines to the scene of a siege. Often, the crew of siege engine, used to break and crack fortresses walls, exceeded 50 soldiers and needed in excess of the same number of oxen to transport to the site of the siege. Commanders would pack these engines in huge cogs (a Medieval type of transport craft limited to coastal sailing) and travel with them for an entire campaign that could last a couple of years and result in the taking of multiple fortresses.
There were two main groups of siege engines direct and indirect fire machines, and within these two groups there existed a multitude of subgroups. The direct fire machine used missile ammunition: arrows, spears, darts, bullets or cannon balls. These weapons generally were five feet in length and capable of piercing the armor of three to four knights in a single shot. The direct fire machine was an anti-personnel weapon used mainly in defensive positions or on ocean craft. Though an attacking army was just as likely to possess multiple engines using this type of ammunition.
The other type of engine was the indirect fire machine this engine hurled or threw projectiles from a distance of up to 250 yards. Indirect fire engines usually threw stones weighing up to 300lbs for the largest engines, though they often hurled dead and decaying animals within the walls as a means of trying to introduce disease. They were also known to throw enemy soldiers and representatives over the walls, this let the fortress commander know that the surrender terms were unacceptable. This was an effective terror tactic.
Engines were used mainly as siege weapons, though there exists a subset within both groups that was portable, to be used on a battle field when two armies converged.
The direct fire engines are a group of siege weapons dating back to the times of antiquity where the Romans perfected the designs and used them with a high degree of proficiency. There are two sub-groups in this category grouped according to how the engines created their energy to fire the missiles. The torsion powered engine created energy by aligning two horizontally aligned arms into a configuration of twisted leather straps or hempen rope. This machine had to be protected from water at all costs, as if the bound configuration got wet it slacked and would not throw. The engine mainly associated with this type of firing system was the 'balista' and rarely used after the downfall of the Romans, but did occur frequently enough in Medieval times to merit mention.
The other type of direct fire engine used tension to create the energy to fire missiles. The energy was created by spanning a bow of wood or steel. The machine using this firing system was called an 'arbalete a tour' and was the dominant direct fire engine during the Medieval period and up until the perfection of the firearm. The 'arbalete a tour' basically was a carriage or post mounted crossbow scaled to fire a five foot shaft. This makes perfect sense because during the Medieval Era the crossbow was the dominant form of infantry missile weapon on the continent. The familiarization with the crossbow made construction relatively simple compared to the balista.
Both of these machines used a wrench or winch to cock the firing mechanism and consisted of crews of two or more. They were often covered and located in a siege tower or accompanied mining cats (type of shelter for soldiers engaging in mining). Another place where this type of engine was common was in marine warfare where size was a limiting factor of the engine and a long shaft packed easier than round stones, and easier to make ammunition for as rocks are not all that common on a boat.
The indirect fire engine (think about the trajectory of the projectile when considering the classification) threw or lobbed a projectile toward a target area. The projectile usually was of stone and directed at the wall of a keep in order break a section creating a breach. These machines also hurled their projectiles over the walls to create mayhem within the safe confines of a castle. In the indirect fire category, again the grouping of machine is determined by the way the engine creates the energy to launch its ammunition, there are two groupings, the torsion engine and the counterpoise or rotating beam engine.
The torsion engine or the 'onager', was another siege engine the Romans developed in late antiquity. The onager is what most people think of when they picture a catapult. The name onager is latin for wild ass and refers to the motion the machine the machine makes after it is fired. Its back end lashes out like an ass kicks due to the excess energy of the machine after firing. Like the direcct fire engine engine of the same sub-group, this engine uses twisted hempen ropes or leather straps to produce the energy to fire its projectile. This engine has a single arm inserted into the twisted rope or strap complex and uses a winch to cock the weapon. The standard delivery device is a cup or bowl mounted on the end of the firing arm facing in the diraction of motion. When the trigger is released, the arm under torsion moves foreward and is stopped at a 90 degree angle to the base by a cross-member perpendicular to the firing arm. the arm stops and the projectile continues moving. The sudden stop delivers the extra energy of the system into the frame of the machine and the 'kick' from the base is where the enging disperses the extra energy. This type of machine had to be reaimed every few throws. The force produced is capabloe of throwing a 25kg projectile (max) 150m (max). The distance is relatively consistant but the extra movement of the machine after firing does not allow it to be consistantly accurate like the trebuchet is capable. This machine was more likely to be present on a ship of war rather than on the siege theater, but the onager could be found often enough being used on land. This engine, too, was sussecptable to water and the torsion complex slackened in this condition. Often in literature this type of engine is referred to as a mangonel. That is the wrong jargon for this machine, the mangonel is a counterpoise engine with a fixed counter weight.
The counterpoise engine or rotating arm engine is subdivided more than any other engine. The counterpoise engine also is more versatile than any other engine. Allowing for changes in size and beam construction, this engine was capable of being classified as a heavy artillery engine or a light anti-personell field weapon. What differentiated the counterpoise engine from all the other siege engines and made it a truely genius was that the construction allowed, barring mistakes in design standards, more energy to be put into the projectile than any other engine. The engine was basically a beam mounted on a fulcrum in such a way that there are two uneven arms. The longer arm was the end attached to a sling that had one end permanently afixed to the beam and the other end of the sling was hooked to the same end of the beam by means of a metal projection that could be altered that the projectile would be released at the angle that would give the maximum distance. Moving down the beam, at the fulcrum an axel is introduced into the beam and attached to a structure for carrying the weight of beam. The energy for firing is produced by acting a force at the end of the short arm after it has been raised to its maximum height. The counterpoise engine is classified according to what produces the force and the degree of hybrid design the engine posesses.
The counterpoise engine is divided into two groups of two depending on how the energy at the end of the short arm is principally delivered. The earliest type of counterpoise engine developed was the traction-type. The traction-type counterpoise engine classification simply means that the energy used to throw the projectile was produced by humans. This machine, the earliest predatecessor to the trebuchet, known is named the pierriere. This machine was first developed in ancient China as an anti-personell field weapon. It was a rotating arm that pivoted on a pole frame with the same sling principle on the long arm ,and on the short arm men pulled on ropes attached to the very end to create the force needed to fire the projectile. This weapon was effective because it was capable of a sustained high rate of fire and was totally portable. The portability and rate of fire made this a good supression weapon when used en masse.
The second of the traction-type sub-group is called a bricole, and is essentially the same as a pierriere, but has the advantage of the innovation of a counterweight attached to the short arm of the beam. The weight allows for a lesser ammount of personell needed to throw the same projectile the same distance. These field weapons were highly efficient for what they did, and as a result of being an excellent supression device they remained in the Medieval artillery arsenal along with the most complex and largest to the counterpoise engines. The evolution of the tension -type counterpoise engine to the counterweight type counterpoise engine took place as this type of engine moved upthrough the central steppes of the Volga River drainage and the availability of wood became more prevalent and allowed for more massive forms of the bricole to be successively built. Along the way the ropes that harnessed the energy humans put into the system were abandoned as the counterweight grewin size and became progressively unsafe to be in the plane the beam traced out as it rotated. It is hypothesised that the Northern Germans and Vikings were the first Europeans to use this type of machine in combat. As early as 700 CE, the Vikings were plying the rivers of now Russian Europe that drained into the Black sea trading furs, amber and woods with the Byzantium Empire and other groups of people through out the Orient and Antonolia, thus becoming the first Germanic peoples to come into contact with this type of artillery.
Again, as a result of the Vikings being seasonally nomadic, if there were tension-type machines present, they would not be exceptionally large as they would take up valuable trading space in the long boats. And because the Vikings used runes principally for burial there would be no record of there being this type of machine being known. Only after the First Crusade is over is there mention in the histories of a counterpoise type engine being built, and in that reference strange as it may seem, a Saxon is called in to build a siege engine for a Frank. Why a Saxon? Because, the Saxons were in a position in Europe to have contact with the Vikings on a regular basis and benefit first from the scarce goods and knowledge brought north by the Vikings finishing the return leg of their inland river trading routes. At this time the Saxons had not migrated to Britain yet and lived on the continent near the coninental coast of the Baltic Sea. And what was this Saxon charged with building?
The second group of counterpoise engines uses a counterweight to supply the principle energy for firing the projectile. This second group of counterpoise engines consists of two machines the mangonel and the trebuchet. The mangonel was a counterweight -type counterpoise engine with a counterweight fixed to the main beam. It would have developed first as this kind of artillery moved north. It was a natural evolution of the bricole, larger trees and a society based on a fuedal ideal allowed larger machines to develop. After the first crusaders saw this machine being used in the Orient and Antonolia, it was natural that it would begin to refine itself to eventually develop a hanging counterweight and the trebuchet is born.
The trebuchet differiantiates itself from the mangonel in that the counterweight is contained within a massive hanging basket attached to the short arm, rether than a fixed weight to the short arm. The hanging weight allowed the mass in the basket to move in a straighter path dowm with a minimal ammount of energy being transferred into the horizontal direction. The size of trebuchet developed in the next century (1150-1250) in Western Europe allowed for counterweights of 20 tons and projectiles up to 300 lbs to be thrown in the first place. This size engine produced effects in which the ramafacations changed the design of newly built castles and changed the inventory of the Medieval artillery armoury to reflect the dominance of the superiority of the trebuchet over the widely used onager, which became relegated to an intermediate artillery piece. As evidence for this, during the Hundred Years War in which the English decimated and brutalized the French, Edward III vassal based out of a region near Calias is reported to having used 26 separate trebuchets to lay siege to a French strong hold a scant two centuries later. There is almost no mention of the mangonel being used, nor the catapult and by 1550 the trebuchet is a reminice of old campaigners.