The Mighty Ballista: Engineering an Ancient War Machine

Originating in the ancient world and used for almost two thousand years, the ballista crossbow was a formidable siege engine designed to launch projectiles, such as stones, bolts, or spears, over vast distances with astonishing accuracy. This is the story of the ballista, one of history’s most formidable weapons of war.

Military History
7 May 2024

The ballista was an ancient engineering marvel that dominated battlefields with its deadly precision and formidable power, reshaping the very art of war. Known as a ‘missile’ or ‘range weapon’, it resembled a large crossbow and was a pivotal innovation in warfare, enabling armies to shape battlefields, breach fortifications and overcome defences from a safe distance. It was so effective it was often seen as the greatest achievement of ancient military engineering. No other weapon of its time could match this ancient bolt thrower for accuracy or destructive power.

Let’s journey back to the ancient world to discover the story of the ballista, how it works, why it became known as the Roman crossbow, and its remarkable longevity as one of the most feared weapons in the history of warfare.

The Origins of the Ballista

Roman siege instruments (Credit: DEA / ICAS94 via Getty Images)

Historically, the roots of the ballista can be traced back to earlier Greek technologies developed around the fourth century BC, probably for the Greek tyrant Dionysius of Syracuse.

Some sources credit the invention – or at least the development – of the ballista to Greek engineer Philo of Byzantium, but that’s far from certain, and there may have been earlier, more rudimentary, versions of the weapon in existence.

The name of the weapon comes from the ancient Greek verb ballein, meaning ‘to throw’. It morphed into ballistes, meaning ‘thrower’ or ‘launcher’, and eventually into ballista.

Believed to have also been referred to as the ballistra, it developed from two weapons, a first century hand-held crossbow called a gastraphetes, and its big brother, the oxybeles, a mounted version of the gastraphetes with a stock and a trigger. However, these were tension machines – described as oversized slingshots – whereby less energy could be transferred to the projectiles, limiting the range of the weapons.

With the invention of the torsion spring came the first ballistae and the ability to fire with incredible power over much further distances. Made of wood, the first versions of the ballista crossbow were unwieldy. They were transported by an army’s baggage train and assembled by a team of men on the battlefield. The weapon became popular under the rule of Philip II of Macedon and his son, Alexander the Great, who further developed it to increase its velocity and brutal power.

How did the Ballista Work?

Ancient Roman ballista (Credit: PHAS via Getty Images)

The ballista, often called an ancient bolt thrower, operates in a similar way to a giant crossbow, using mechanical principles to launch projectiles over long distances.

Frame & Base
The ballista has a large, sturdy wooden frame mounted on a base, which allows it to be aimed and provides stability.

Torsion Springs
At the core of the weapon are torsion springs, which are bundles of twisted ropes or sinews. These springs are located in two cylindrical chambers on either side of the arm of the ballista.

Throwing Arm
A long throwing arm is inserted through the torsion springs. When the arm is pulled back, it twists the springs, storing a vast amount of potential energy.

Trigger Mechanism
The arm is held in place by a trigger mechanism. When the trigger is released, the torsion springs rapidly unwind, converting the stored potential energy into kinetic energy.

This unwinding causes the throwing arm to accelerate forward, propelling the projectile (such as a stone, bolt or spear) towards its intended target at a very high speed and with remarkable precision.

Who Used the Ballista?

Roman battery ballista from Trajan's Column (Credit: Sepia Times via Getty Images)

The ancient Greeks were the first to use the ballista, but it was the Romans who extensively adopted and adapted it, perfecting it to such an extent that it became a standard piece of equipment in Roman siege arsenals, renowned for its reliability and power in battlefield and siege conditions.

It was used at the end of the Roman Republic during Julius Caesar’s conquest of Gaul, his first incursion into Britain in 55 BC, and in many of the Roman Empire’s battles. In fact after Caesar’s reign as dictator, the ‘Roman crossbow’ became a permanent fixture in the Roman army.

The Romans modified the ballista over the years to decrease weight, and improve efficiency and power. By the fourth century, the biggest ballistae in the Roman arsenal could propel a projectile hundreds of metres. Around the same time, during the Roman occupation, more than twenty towers were built around the old London Wall to provide them with permanent placements.

Later versions of the ballista crossbow included a cart-mounted version called the carroballista which could be more easily moved around a battlefield instead of being dismantled and rebuilt with every advance or retreat.

A portable version called the cheiroballistra (Greek) and manuballista (Latin) – translated to ‘hand ballista’ which can be seen carved into Trajan’s Column in Rome – were made for close-quarters combat. It’s also thought that a repeating weapon capable of firing eleven bolts per minute was built. Called a polybolos, archaeologists have yet to unearth one and its existence remains a matter of conjecture.

The Decline of the Ballista Crossbow

A type of Roman ballista (Credit: Stefano Bianchetti via Getty Images)

Despite its prominence in ancient and mediaeval conflicts, the ancient bolt thrower eventually saw a decline as technologies evolved. Its use started to decline during the later Roman Empire period due to advancements in other siege technologies such as onagers, which were simpler to produce in an era of continually-declining imperial resources.

By the Middle Ages, influenced in part by changes in military tactics and fortification styles, gunpowder-based weapons such as cannons began to dominate, offering greater firepower and range, effectively rendering siege engines such as the ballista obsolete.

The Enduring Impact of the Ballista

A small Roman ballista (Credit: nikolay100 via Getty Images)

The ballista crossbow was both a remarkable feat of ancient engineering and a brutal weapon of destruction. Its development and deployment across several centuries of ancient warfare underscores its significance in shaping the strategies and outcomes of historical conflicts. Even as the age of the ballista gave way to more practical alternatives and the eventual advancement of gunpowder weaponry, the legacy of this powerful siege engine highlighted a period when mechanical engineering was as crucial on the battlefield as the bravery of the soldiers themselves.


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