A castle was designed to be impossible to enter. With walls five meters thick, towers that allowed flanking fire, and gatehouses rigged with murder holes, a well-stocked fortress could theoretically hold out indefinitely against a superior force. Yet, history is full of successful sieges. Château Gaillard, Rochester Castle, Krak des Chevaliers—all fell.
How?
The art of poliorcetics (siege warfare) was a grim contest of engineering, patience, and brutality. It was rarely about charging the walls with ladders (movies love this; real soldiers hated it). Instead, it was a systematic dismantling of the defenses using five primary methods: Starvation, Deception, Mining, Bombardment, and Storming.
1. Starvation: The Waiting Game
The most common siege tactic was also the most passive. If you cannot get in, make sure nothing else can either. A siege army would surround the castle, cutting off supply lines. They would build their own ring of fortifications (circumvallation) facing inwards to trap the defenders, and sometimes a second ring (contravallation) facing outwards to protect themselves from relief armies.
The Psychology of Hunger
Starvation was a weapon of terror. As food stocks dwindled, the castle commander faced a terrible choice.
- The “Useless Mouths” Dilemma: To save food for the fighting men, commanders would often eject women, children, and the elderly from the castle.
- Château Gaillard (1203-1204): In a horrific incident, King Philip II of France refused to let these refugees pass through his siege lines. The English commander refused to let them back into the castle. Hundreds of civilians were trapped in the freezing no-man’s-land between the walls and the trenches, where they starved to death over the winter.
2. Mining (Sapping): The Invisible Enemy
Before gunpowder, the most feared weapon was a pickaxe. Sapping involved digging a tunnel beneath the castle walls. Ideally, the tunnel would start far away, hidden from view.
The Technique
- Miners dug a tunnel supported by wooden props.
- Once they reached the foundation of a wall or tower, they excavated a large cavity, replacing the stone foundation with wood.
- They filled the cavity with brushwood, pig fat, and pitch.
- They set it on fire.
- As the props burned, the tunnel collapsed, and the wall above it fell into the crater, creating a breach.
The Pig Fat of Rochester (1215)
The most famous example occurred at Rochester Castle. King John besieged the rebel barons inside. His miners successfully undermined one corner of the massive stone keep. To ensure a particularly hot fire, John ordered the Sheriff of Kent to send “forty of the fattest pigs of the sort least good for eating” to be packed into the tunnel. The fat fueled an inferno that brought the corner tower crashing down.
Counter-Mining
Defenders were not helpless. They would place bowls of water on the ground near the walls. If the water rippled, they knew miners were digging beneath. They would then dig counter-mines to intercept the enemy tunnel. When the two tunnels met underground, terrifying hand-to-hand combat ensued in the pitch black, fought with knives and picks.
3. Bombardment: The Physics of Destruction
If you couldn’t go under the wall, you had to smash it.
The Trebuchet
The trebuchet was the king of siege engines. Unlike the catapult (which used tension), the trebuchet used a massive counterweight to use gravity as an energy source.
- Physics: A heavy box filled with earth or lead (often 10-20 tons) was raised. When released, it swung a long arm. A sling attached to the arm whipped around, releasing a projectile at the top of the arc.
- Warwolf: At the siege of Stirling Castle (1304), Edward I built the largest trebuchet in history, named “Warwolf.” It reputedly stood 3 or 4 stories high. The mere sight of it convinced the Scots to surrender. Edward refused the surrender because he wanted to test his new toy. It successfully smashed a hole in the curtain wall.
Biological Warfare
Engines didn’t just throw stones. They threw dead horses, diseased cows, and even the severed heads of captured messengers. This was psychological warfare and early biological warfare, intended to spread disease (like dysentery) inside the cramped castle.
4. Assault: The Storming of the Walls
Storming a castle was a last resort. It was costly in human life (“the forlorn hope”). However, specific machines were designed to aid the assault.
The Siege Tower (Belfry)
A massive wooden tower on wheels, covered in wet animal hides to prevent fire. It was pushed up to the walls. Inside, ladders allowed soldiers to climb to the top, where a drawbridge would drop onto the castle battlements, allowing knights to storm across on level footing.
- The Defense: Defenders would pile dirt at the base of the wall to stop the wheels, or use a “batter” (sloped wall base) so the tower couldn’t get close enough for the bridge to drop.
The Battering Ram
A huge tree trunk, often tipped with iron, suspended from a frame within a covered shed (the “cat” or “mouse”). It was slammed repeatedly against the castle gates or a weak section of wall.
- The Defense: Defenders would lower thick mattresses or wooled ropes over the wall to cushion the blows. Alternatively, they would use giant grappling hooks (cranes) to grab the ram and tip it over.
5. Deception and Treachery
Often, the easiest way to take a castle was to find a traitor. A bribed guard opening a postern gate at night was worth more than ten trebuchets. Alternatively, disguise was used. Small groups of soldiers dressed as monks, merchants, or peasants would approach the gates, only to pull out swords once inside and hold the gate open for the main army hiding nearby.
Chemical Warfare: Greek Fire and Quicklime
Medieval warfare was not just kinetic; it was chemical. Long before mustard gas, siege engineers used terrifying substances to burn and blind.
Greek Fire (The Byzantine Secret)
The most famous chemical weapon was Greek Fire, a liquid incendiary composition used by the Byzantine Empire. Its exact recipe is lost to history, but it likely contained naphtha and quicklime.
- Properties: It burned on water. In fact, water often intensified the flames. It stuck to everything—wood, stone, flesh.
- Deployment: It was siphoned through bronze tubes (primitive flamethrowers) or thrown in clay pots (grenades).
- Psychological Impact: The noise was described like thunder, and the smoke turned day into night. Crusaders described it as a dragon flying through the air.
Quicklime (The Blinding Dust)
Defenders often threw Quicklime (calcium oxide) down from the machicolations. When quicklime contacts moisture (sweat, tears, or the mucus membranes of the eyes/lungs), it creates a violent exothermic reaction. It essentially boils on the skin and blinds the victim. It was a cruel, cheap, and effective weapon against soldiers climbing ladders.
Case Study: The Siege of Acre (1189–1191)
To understand the complexity of a major siege, we look to the Siege of Acre during the Third Crusade. It was a “double siege.”
- The Situation: Guy of Lusignan (Crusader King) besieged the Muslim garrison in the city of Acre.
- The Twist: Saladin (Muslim Sultan) arrived with his army and besieged Guy’s besiegers from the outside.
- The Stalemate: For two years, the armies were locked in concentric rings. The Crusaders were starving in their trenches while trying to starve the city.
- The Technology: Both sides used massive trebuchets. The Crusaders built huge siege towers, which the Muslim defenders destroyed using jars of naphtha.
- The Resolution: The arrival of Richard the Lionheart and Philip II of France with fresh supplies and massive engines finally cracked the walls. The city surrendered, ending one of the deadliest sieges in medieval history.
The Logistics of the Besiegers
We often focus on the castle, but the besieging army faced a logistical nightmare.
- Camp Hygiene: An army of 10,000 men produces tons of waste daily. Dysentery (“the bloody flux”) killed far more soldiers than arrows. At the Siege of Harfleur, Henry V lost a third of his army to disease before even fighting a battle.
- Supply Lines: The besiegers had to eat. They would ravage the surrounding countryside for miles (“foraging”). Once they had eaten everything within a day’s ride, they had to rely on complex supply chains. If the castle could hold out until winter, the besieging army would often disintegrate due to lack of food and shelter.
- Boredom and Discipline: Keeping thousands of armed men disciplined while they sit in mud for months was a massive challenge. Gambling, drinking, and fighting were rampant in siege camps.
The End of an Era: The Mons Meg
The invention of the cannon changed everything. For centuries, the advantage lay with the defender. A stone wall was stronger than any stone thrown at it. Gunpowder reversed this physics. A cast-iron cannonball fired at high velocity could shatter masonry. The famous bombard Mons Meg (now at Edinburgh Castle) could fire a 175kg stone ball over 3km. Against such kinetic energy, vertical walls were a liability. Collapse was inevitable. This led to the design of the Star Fort, low and thick, designed to absorb impact rather than resist it.
Conclusion
Siege warfare was a spectrum of horror. It ranged from the slow, quiet agony of starvation to the thunderous crash of the trebuchet. It was a test of logistics as much as bravery. A castle was only as strong as its well, its granary, and the morale of its garrison. As the saying went: “A castle built on a hill is strong; a castle built on gold is invincible.”