Meet the Trebuchet, the Castle-crushing Catapult of the Middle Ages

Before gunpowder was popularized in the mid-14th century, there were no canons that could launch heavy lead balls through enemy bodies and walls. But that didn't stop creative warfighters from devising ways to toss projectiles at each other. One of the most effective was the catapult, a device that uses a spring-loaded arm or a heavy counterweight to hurl large objects over great distances.

Gurstelle says there are three general types of catapults:

The very earliest trebuchets, like those first used in China and later in Europe in the early Middle Ages, were people-powered, meaning the lever arm of the catapult was swung by a group of soldiers pulling on a rope. But the real innovation in trebuchet technology came in the 12th-century with the advent of the counterweight trebuchet.

"It's all really basic physics at a fundamental level," says Michael Fulton, a history professor at Langara College in British Columbia and author of "Siege Warfare During the Crusades." An elevated basket is weighted with hundreds or even thousands of pounds of rocks — that's the counterweight. When the basket is dropped, it pulls down on a rope connected to the short end of a long lever arm that swings on an axel.

"As the short end of the lever is pulled down, the long end rises at a proportionately greater rate," says Fulton. "When you add a sling to the end of the arm, you force the projectile to travel even farther during the same amount of time, which adds to your rate of acceleration."

Gurstelle has built plenty of trebuchets, including a DIY design using wood and PVC that he named "Little Ludgar" after Edward's trebuchet that leveled the Scots.

"The longer that lever and the heavier the weight, the farther the projectile goes," says Gurstelle, noting that the counterweight has to weigh approximately 100 times the object you're trying to throw. Gurstelle once made a large trebuchet with a 500-pound (226-kilogram) counterweight that was still only powerful enough to launch a small cantaloupe.

During the Middle Ages, the construction of fortified cities led to a new type of military campaign — the siege. Laying siege to a walled city required new war machines like battering rams for splintering thick doors and siege towers for breaching high walls. But one of the earliest and most innovations was the trebuchet.

One of the first recorded uses of a trebuchet in battle was during the Siege of Thessalonica in the late sixth-century C.E. Thessalonica was a Byzantine stronghold under attack by the Avars, a collection of Central Asian tribes who used a people-powered trebuchet that was likely inspired by ancient Chinese weaponry.

Those primitive "traction" trebuchets could only launch small projectiles and functioned as anti-personnel weapons, says Fulton, not castle killers.

"Traction trebuchets were like an archer on steroids," says Fulton. "You're definitely not smashing down solid walls in the early Middle Ages." That happens in the 13th century, when counterweight trebuchets were being built at larger and larger scales all across Europe.

Those truly massive trebuchets would be constructed off-site and then assembled on the battlefield itself. While a counterweight trebuchet could toss a boulder over a castle wall, there were definitely trade-offs. For one, it took a really long time to reload the counterweight. Fulton says that the smaller traction trebuchets could fire up to four shots a minute, while the biggest trebuchets were lucky to get off one shot every half-hour.

Catapults and trebuchets were not limited to firing conventional projectiles like stones and lead balls. According to one 14th-century account, the Mongols used their catapults to launch plague-ridden corpses, an early type of bioweapon, into the medieval city of Caffa in modern-day Ukraine. Other stories tell of dead horses being slung by trebuchet over castle walls to sicken the enemy with the stench.

Fulton, who has witnessed the forces unleashed during the throwing sequence of a large trebuchet, is skeptical about the accuracy of such accounts. "If you try to put something organic into one of those slings, chances are it's going to be ripped apart before you can throw it effectively," he says.

Fulton has more confidence in the tales of human heads being lobbed back and forth by trebuchets at the Siege of Nicaea in 1097, during the First Crusade.

"That was more psychological than biological," says Fulton.

In the opening scene of the Netflix movie "Outlaw King," Edward I unleashes his Warwolf on Stirling Castle with a fabulous explosion of what he calls "Greek fire." Did such a thing exist?

Gurstelle explains that Greek fire was a secret weapon of the Byzantine empire that was like "ancient napalm."

"Once you lit it and threw it, you couldn't put out the flames with water and it would burn very intensely," says Gurstelle, adding that the recipe for Greek fire — pine tar, sulphur, naturally occurring petroleum — was "lost in the sands of time."

Fulton agrees that Greek fire was a popular Byzantine incendiary weapon, especially for naval attacks, but doubts that Edward or anyone else was launching Greek fire bombs from trebuchets with any regularity. It was more likely that castle defenders would try to fire incendiaries at the trebuchet to burn the weapon to the ground.

Even if Edward's legendary trebuchet only launched rocks, there simply was no siege weapon that was as terrifying to the enemy and as entertaining to the troops.

"At a fundamental level, you're not going to build these engines unless they have value, but there is value in that intimidation factor," says Fulton. "In general, kings like to have big things they can show off."