In the annals of medieval warfare, Catapults and Trebuchets stand out as formidable siege weapons. Crafted to hurl projectiles such as rocks, flaming rags, or disease-ridden objects, they were the scourge of castle walls. As these weapons proved their destructive might, fortifications responded by fortifying walls and raising towers, sparking a technological arms race in siege engineering. This led to a proliferation of catapult and trebuchet designs, each striving for superiority in both form and function. Though consigned to the annals of history in modern warfare, these ancient marvels continue to intrigue us with their ingenuity. They also offer a fascinating study in engineering principles, structural forces, and energy transmission.

Catapult Basics

A catapult is a ballistic (the field of engineering of launching and flight) device used to launch a projectile a great distance without the aid of gunpowder or other propellants – particularly various types of ancient and medieval siege engines. A catapult uses the sudden release of stored potential energy to propel its payload. Most convert tension or torsion energy that was more slowly and manually built up within the device before release, via springs, bows, twisted rope, elastic, or any of numerous other materials and mechanisms.

In use since ancient times, the catapult has proven to be one of the most persistently effective mechanisms in warfare. In modern times the term can apply to devices ranging from a simple hand-held implement (also called a "slingshot") to a mechanism for launching aircraft from a ship.

Types of Catapults

Fixed Weight

A fixed weight catapult is the simplest of catapults. This simply raises a heavy weight on the fulcrum of a throwing arm. The potential energy of the lifted weight catapults the object in the bowl. This catapult is limited in that the fixed weight must be very heavy to launch anything that has any significant weight as well. 


A torision catapult works similar to a fixed weight catapult in that the object is to fling the bowl and payload as fast as possible. However, instead of weight, the potential energy for this is stored in a spring or torqued band that releases its energy like a stretched rubber band. 


The Ballista is a type of catapult that looks the least like what we imagine to be a catapult. It looks like an oversized crossbow and behaves very similarly as well. Potential energy is stored as the arms tension and bend. We released, the payload launches out the machine similar to a slingshot. We may be familar with the viral video of a Ballista being used going very wrong.  


A Springald Catapult combines the design of the ballista and a traditional catapult. This machine has its potential energy stored in tensioned material that will bend and store energy but not break. When released, the arms straighten out and put on a cord that is wrapped around the fulcrum of the catapult causing the arm to spin rapidly. Leonardo Da Vinci designed this particular model. 

Trebuchet Basics

A trebuchet is a compound machine that makes use of the mechanical advantage of a lever to throw a projectile. They are made primarily of wood, usually reinforced with metal, leather, rope, and other materials. They are usually immobile and must be assembled on-site, possibly making use of local lumber with only key parts brought with the army to the site of the siege or battle.

A trebuchet consists primarily of a long beam attached by an axle suspended high above the ground by a stout frame and base, such that the beam can rotate vertically through a wide arc. A sling is attached to one end of the beam to hold the projectile. The projectile is thrown when the beam is quickly rotated by applying force to the opposite end of the beam. The mechanical advantage is primarily obtained by having the projectile section of the beam much longer than the opposite section where the force is applied – usually four to six times longer.

The difference between counterweight and traction trebuchets is what force they use. Counterweight trebuchets use gravity; potential energy is stored by slowly raising an extremely heavy box (typically filled with stones, sand, or lead) attached to the shorter end of the beam (typically on a hinged connection), and releasing it on command. Tension trebuchets use human power multiplied with simple machines and forces like tension or spings. Both require manpower to set and reset. Energy, usually in the form of human power, has to be used to create these high amounts of potential energy. Further increasing their complexity is that either winches or treadwheels, aided by block and tackle, are typically required to raise the more massive counterweights. So while counterweight trebuchets require significantly fewer men to operate than traction trebuchets, they require significantly more time to reload. In a long siege, reload time may not be a critical concern.

When the trebuchet is operated, the force causes rotational acceleration of the beam around the axle (the fulcrum of the lever). These factors multiply the acceleration transmitted to the throwing portion of the beam and its attached sling. The sling starts rotating with the beam, but rotates farther (typically about 360°) and therefore faster, transmitting this increased speed to the projectile. The length of the sling increases the mechanical advantage, and also changes the trajectory so that, at the time of release from the sling, the projectile is traveling at the desired speed and angle to give it the range to hit the target. Adjusting the sling's release point is the primary means of fine-tuning the range, as the rest of the trebuchet's actions are difficult to adjust after construction.

The rotation speed of the throwing beam increases smoothly, starting slow but building up quickly. After the projectile is released, the arm continues to rotate, allowed to smoothly slow down on its own accord and come to rest at the end of the rotation. This is unlike the violent sudden stop inherent in the action of other catapult designs such as the onager, which must absorb most of the launching energy into their own frame and must be heavily built and reinforced as a result. This key difference makes the trebuchet much more durable, allowing for larger and more powerful machines.

Types of Trebuchets

All trebuchets have a few similar components that they must have to be considered a trebuchet. The primary component that makes a trebuchet unique compared to a traditional catapult is its sling. The sling rotates around the arm adding a second fulcrum and an additional mechanical advantage. This allows trebuchets to launch thier payload way further and with more force than a traditional catapult. 


Counterweight Trebuchets use heavy weights that have to be hoisted high just like the fixed weight catapult. These weights can be fixed or swinging in a basket like the picture shown above. The benefit of a basket is that it can be loaded with stones, and other movable items to adjust the weight. 


Torsion Trebuchets use the tension and potential energy of a wound up band. In medieval times this would have been robe or other fibers twisted by crank, but in modern times this can be rubber or other synthetic elastic material that can store a tremendous amount of potential energy. 

Floating Arm

The Floating Arm Trebuchet works very similarly to the counterweight trebuchet in that the weight is fixed to the end of the arm and lifted high. However, the fulcrum is not fixed to the frame and moves. This allows the arm and counterweight to fall further and faster increasing the energy of the throw.  


The Merlin Trebuchet is one of the most complicated designs the trebuchet. It uses a weight like the counterwieght trebuchet, but it falls over a variable radius canter axle that accelerates as it rotates.

Catapult vs Trebuchet

When comparing catapults and trebuchets, both are ancient siege weapons used for hurling projectiles, yet they operate on distinct principles.

Catapults, typically smaller and simpler in design, rely on tension, torsion, or gravity to launch objects. They were prevalent in ancient warfare, with variations like the ballista and mangonel.

In contrast, trebuchets are larger and more intricate machines. They feature a pivoted arm with a heavy counterweight on one end and a sling for the projectile on the other. As the counterweight drops, it generates powerful rotational force, launching projectiles with significant force and accuracy.

While catapults are effective for shorter distances and simpler mechanisms, trebuchets excel in launching heavier projectiles over longer ranges with greater precision. This distinction makes trebuchets particularly formidable weapons during the Middle Ages, dominating sieges and battles with their superior firepower and range.

Build a Catapult or Trebuchet

When building any project, it is important to follow the six steps of the Engineering Design Process. They will be aligned to the steps you should follow when designing and building your model trebuchets.


Identify who and what the final design is intended for and what constraints need to be addressed.

Decide on what your final constraints are with this project. This can include material, size, or design around what you want to build. For a model, consider throwing a ping pong ball, grapes, or marshmallows!


Look for similar solutions to similar problems that already exist and see how they may be helpful for your design. 

Medieval Siege weapons have been around for hundreds of years. A lot of thought and design already went into these machines. Research what is out there and is used successfully. 


Break the problem down into components to help guide you in developing multiple ideas that may or may not work as a design solution. 

Whenever designing anything, you should be prepared with multiple ideas and sketches and be prepared to move forward with them based on need. 


Pick one design that you want to develop further that you feel addresses the problem the best and meets all the constraints.

Before you spend time and money on material, you should make sure your design is fleshed out. In a classroom, this can be done with a hand-drawn orthographic projection, or 3D modeled on software. 


Check your design solution vs the problem and all of its constraints. 

Testing your projects is time-consuming but worth it. Trebuchets especially are very finicky because of their slings, but once tuned in are highly efficient machines. Be critical of this process and fix what does not work.


Review your test results and analyze what went right or wrong with your solution. Then iterate through the whole process again. 

If you have the time for a redesign and rebuild, do it. Small tweaks and changes can have a massive change in output efficiency.