The Writer’s Guide to Siege Engines
Besieging a castle or a city inherently has a great amount of built-in tension that a writer can use to fantastic effect. If you want to read besieging a castle, I recommend my last article, The Writer’s Guide to Besieging a Castle. Today, we are focusing on siege engines, a general term that includes catapults, trebuchets, and battering rams. My focus will be medieval Europe since I have limited knowledge of siege engines outside of this location and period.
As always, magic is the exception to the rules. Because magic.
The earliest type of siege engine is the siege tower, also known as a belfry.  This might seem strange at first because it is a passive piece of equipment. However, siege towers were highly effective against walls and other fortifications. They were enclosed towers on wheels that were maneuvered into place against a wall. Then a gangplank was lowered, bridging the way to the top of the wall, and allowing soldiers to rush across. They could also have archers on the top. The benefit of siege towers was that it protected the soldiers inside from arrows and other projectiles and were sometimes covered in fresh animal hides to prevent it from being set on fire.  It allowed many of them to gain the battlements at one time.
The oldest siege towers were used in the 9th century by the Neo-Assyrian Empire. The Romans used them to great effect, such as at the siege of Masada in 73-74 CE.  The Greeks and ancient Chinese also used them.
The battering ram is well known in history and fiction. It comes in several forms, the simplest being a large tree trunk or log cleaned of branches that is carried by a group of people and slammed against a gate or other barrier (think “Beauty and Beast”). Later, the battering ram would be suspended from a wheeled frame by ropes or chains. It would be pulled back then allowed to swing forward. Eventually, a roof was added to the frame to protect the wielders from arrows, rocks, or whatever other nasty surprises a besieged fortress could muster.
There is some evidence that battering rams date back to Bronze Age Egypt.  Many cultures used them throughout history including the Carthaginians, Assyrians, ancient Greeks including Alexander the Great, the Romans, and most countries in Europe during the Middle Ages.
The catapult is the first missile thrower on our list. It consists of an arm ending in a bucket, into which the missile is placed. A bundle of twisted ropes, sinew or other materials was at the base of the arm. As the arm was pulled back, it twisted this bundle further, leading to a build up of torque and energy since the twisted ropes were trying to straighten. When the arm is released, the ropes fling it forward at speed, propelling the missile in the bucket. However, this type of torsion system can only produce a certain amount of throwing power. It wasn’t enough to be effective against stone walls and was mainly used against personnel.
The earliest catapults are from 4th century China, but they were also used by the ancient Greeks and Romans. 
The ballista is basically a giant crossbow and operates on the same principle. Tension is created by pulling back on the string, usually with a winch and rachet system, and flexing the bow arms. When the string is released, the missile is propelled forward. Usually, either giant crossbow bolts or stones were used. As a result of the bigger size, everything about a ballista was scaled up from its smaller cousin. Generally, it was used as an anti-personnel weapon.
The earliest ballistae were developed by the ancient Greeks although the Romans used it as inspiration for the smaller scorpion.
The trebuchet was the pinnacle of siege engine technology and could dish out an incredible amount of energy and destruction. It wouldn’t be until the advent of artillery that it’s destructive power would be surpassed. The trebuchet has a long throwing arm with a sling attached to it. The throwing power was achieved either with a counterweight or traction. A counterweight trebuchet has a large bucket on the opposite end of the throwing arm that is weighed down, commonly with rocks. The arm was pulled down, either with brute form or a winch. When it was released, the counterweight dropped, swinging the arm, and attached sling up. The traction trebuchet relied on a large group of people hauling together on ropes to provide the energy to swing the arm. Please be aware that “counterweight trebuchet” and “traction trebuchet” are modern terms and we have no evidence they were used in history. 
Trebuchets were also incredibly versatile, and we have records of them being used to attack and defend fortifications as well as on ships. It was the first siege engine that could successfully take down castle walls. They could also fire a variety of missiles including stones or even bombs of lime and sulfur such as were fired during the Battle of Caishi in 1161. 
The traction trebuchet is thought to have originated in China as early as the 4th century.  However, it’s believed that the biggest trebuchet in history was Warwolf, which was built on the order of King Edward I of England to fight the Scots. It could reportedly throw rocks weighing up to 298 pounds (135 kg) a distance of 660 feet (200 m). 
I hope this was helpful. Let me know if you have any questions or suggestions by using the Contact Me form on my website or writing a comment. I post every Friday and would be grateful if you would share my content.
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Copyright © 2021 Rebecca Shedd. All rights reserved.
 Castle: Stephen Biesty's Cross-Sections. Dorling Kindersley Pub (T); 1st American edition (September 1994). Siege towers were invented in 300 BC. ISBN 978-1-56458-467-0.  Duncan B. Campbell, "Capturing a desert fortress: Flavius Silva and the siege of Masada", Ancient Warfare Vol. IV, no. 2 (Spring 2010), pp. 28–35. The dating is explained on pp. 29 and 32.  "Siege warfare in ancient Egypt". Tour Egypt. Retrieved 23 May 2020. ...we find a pair of Middle Kingdom soldiers advancing towards a fortress under the protection of a mobile roofed structure. They carry a long pole that was perhaps an early battering ram.  Chevedden, Paul E.; et al. (July 1995). "The Trebuchet". Scientific American: 66–71. Original version.  Purton, Peter (2009), A History of the Early Medieval Siege c.450-1200, The Boydell Press  Needham, Joseph (1987). Science and Civilisation in China: Military technology: The Gunpowder Epic, Volume 5, Part 7. Cambridge University Press. p. 166. ISBN 978-0-521-30358-3.  Chevedden, Paul E.; et al. (July 1995). "The Trebuchet". Scientific American: 66–71. Original version.  "The largest trebuchet ever built: Warwolf in the Siege of Stirling Castle / thefactsource.com". Retrieved 2020-03-25.