Mongol Empire - Part 9: Siege Warfare Revolution: How Nomads Learned to Take Walled Cities

The Mongols were horsemen. Their entire civilization was built around mobility – following herds, raiding rivals, moving with the seasons. They lived in felt tents that could be packed in an hour. They fought from horseback with composite bows. Everything they knew screamed: keep moving.

Cities were the opposite. Walls, moats, towers, gates. Designed specifically to stop mobile enemies. For millennia, nomadic peoples had broken against fortifications like waves against rock.

Yet within a single generation, the Mongols became history’s most effective siege operators. By the mid-13th century, no city in the world could resist them. How?

The answer reveals something profound about organizational learning – and the difference between building capabilities versus acquiring them.

The Problem: A Civilization Behind Walls

When Genghis Khan unified the Mongol tribes around 1206, he faced a strategic reality: all the wealth was behind walls.

The Geography of Power

The great centers of wealth and power were urban:

• Chinese dynasties with massive walled cities
• Persian/Islamic empires with fortress networks
• European kingdoms with castles and fortified towns
• Russian principalities with wooden and stone kremlins

The Mongol way of war – rapid movement, envelopment, harassment – was irrelevant against walls. You can’t shoot arrows at a stone tower. You can’t outflank a moat.

The Traditional Steppe Solution

Historically, nomadic confederations had three options:

1. Raid and retreat – Take what’s outside the walls, leave the cities alone
2. Long blockade – Starve cities out (but nomads get restless)
3. Buy or diplomacy – Trade access to get some wealth without fighting

None of these built empires. The wealth stayed behind walls. The nomads stayed outside.

Genghis Khan chose a fourth option: learn siege warfare.

The Acquisition Strategy

The Mongols could have spent decades developing siege technology from scratch. Instead, they did something smarter: they acquired it wholesale.

Capture the Engineers

From their first campaigns against the Jin Dynasty in northern China, the Mongols systematically:

• Identified valuable specialists among prisoners – engineers, siege weapon operators, crossbow makers
• Spared them while executing other captives
• Integrated them into Mongol forces immediately

These weren’t slaves. They were recruited specialists offered:

• Protection from harm
• Continued practice of their trade
• Better treatment than many received from their original rulers
• Rewards for successful sieges

The Technology Transfer Pipeline

As Mongol conquests expanded, so did their engineering corps:

Each conquest brought new capabilities. The Mongol siege train became an international amalgamation of the best techniques from every conquered people.

The Learning Organization

Acquiring technology is only half the challenge. The Mongols excelled at institutionalizing what they learned.

Formalized Knowledge Transfer

• Siege specialists trained Mongol officers in theory and practice
• Standard procedures developed for different fortification types
• After-action reviews identified what worked and what didn’t
• Lessons disseminated across all Mongol armies

The Siege Corps Structure

By the 1230s, the Mongols had a dedicated siege corps (poliorcetes) that included:

• Engineering specialists (various ethnicities)
• Weapons crews for catapults and ballistas
• Mining teams for undermining walls
• Incendiary specialists for fire attacks
• Supply and logistics for the heavy equipment

This corps moved with Mongol armies, providing siege capability wherever needed.

Continuous Improvement

Each siege refined techniques:

The 1258 siege of Baghdad represented the culmination – a complex operation combining bombardment, mining, assault, and psychological warfare into a seamless whole.

Siege Techniques: The Mongol Toolkit

1. Catapults and Trebuchets

The Mongols deployed three types:

• Traction trebuchets – Crew-pulled, rapid fire, lighter
• Counterweight trebuchets – Heavier, more powerful, longer range
• Hybrid designs – Combined elements for specific needs

At major sieges, the Mongols would deploy hundreds of catapults simultaneously, creating continuous bombardment that:

• Destroyed wall sections
• Killed defenders on ramparts
• Prevented repairs
• Demoralized the garrison

2. Mining Operations

Specialized teams would:

• Dig tunnels under walls
• Shore them with wooden supports
• Fill with combustibles
• Burn the supports, collapsing the tunnel and the wall above

This required engineering knowledge the Mongols didn’t have – but their captured specialists did.

3. Assault Towers and Rams

Traditional siege equipment was adapted:

• Siege towers on wheels to approach walls
• Covered battering rams for gates
• Scaling ladders (often using captured civilians to carry them)

4. Incendiaries

The Mongols employed various fire weapons:

• Naphtha (petroleum) launched by catapult
• Explosive bombs (Chinese invention)
• Fire arrows in mass volleys
• Incendiary animals (legend has it they used burning camels)

The Human Weapon: Forced Labor

Perhaps the most disturbing Mongol siege innovation was the systematic use of prisoners.

The Hashar

Captured populations from previously conquered cities were forced to:

• Carry siege equipment
• Fill moats with earth and debris
• Assault walls as the first wave
• Serve as human shields for Mongol troops

This served multiple purposes:

• Preserved Mongol lives – The casualties were others’ citizens
• Demoralized defenders – Who had to kill their own countrymen
• Exhausted defenses – Before the Mongol assault began
• Created terror – Ensuring future cities knew what awaited them

The Moral Calculus

This practice was brutal by any standard. It was also grimly effective. Cities that resisted faced:

• Their own people used as cannon fodder
• Mass enslavement of survivors
• Complete destruction of the city

Cities that surrendered faced:

• Preservation of most citizens
• Relatively orderly occupation
• Integration into the Mongol system

The incentives were designed to make resistance unthinkable.

Terror as Force Multiplier

The Mongols understood that reputation could win battles before they started.

The Surrender Calculus

After each major siege, the Mongols ensured survivors spread the story:

• Executed cities that resisted
• Spared (relatively) cities that surrendered
• Made examples designed to terrify future targets

The Algorithm

A city receiving Mongol envoys faced a clear calculation:

This “surrender calculus” traveled faster than Mongol armies. By the time forces arrived at many cities, the population was already pressuring leaders to submit.

The Reputation Investment

Early massacres were investments in terror that paid dividends later:

• Bukhara (1220) – Mass execution after brief resistance
• Samarkand (1220) – Partial massacre after brief defense
• Nishapur (1221) – Complete destruction (walls leveled, gardens plowed)
• Herat (1221) – Total massacre (reportedly 1.6 million killed – likely exaggerated but massive)

After these examples, many cities surrendered without a fight. The terror investment reduced future siege requirements dramatically.

Case Study: The Fall of Baghdad (1258)

The siege of Baghdad represents the Mongol siege system at its peak.

The Target

Baghdad was the seat of the Abbasid Caliphate – the symbolic heart of Islam. It had:

• Massive walls and fortifications
• A population of perhaps 1-2 million
• The best engineers and defenders available
• 500 years of accumulated wealth

The Preparation

Möngke Khan sent his brother Hülagü with:

• Perhaps 150,000 Mongol and allied troops
• Chinese engineers and siege specialists
• Christian allies from Georgia and Armenia
• Over 1,000 siege engines

The Execution

The siege combined every Mongol technique:

1. Investment – Complete encirclement of the city
2. Bombardment – Continuous catapult fire
3. Mining – Engineers undermined key wall sections
4. Flooding – Irrigation dams broken to fill moats
5. Psychological warfare – Repeated demands for surrender

The siege lasted about three weeks. When walls were breached, the city was systematically sacked.

The Aftermath

• The last Abbasid Caliph was executed
• The population was massacred or enslaved
• Libraries containing centuries of knowledge were destroyed
• The city wouldn’t recover for centuries

Baghdad’s fall demonstrated that no city – no matter how large, wealthy, or symbolically important – could resist the Mongol siege system.

What the Mongols Teach About Learning

The Mongol siege revolution wasn’t about genius invention. It was about systematic acquisition, integration, and improvement. The lessons apply far beyond Military and Logistics:

1. Buy Before Build

The Mongols didn’t reinvent siege warfare – they acquired it. They found the best engineers, captured them, integrated them, and learned from them.

Modern parallel: Companies that acquire capabilities through M&A or hiring often outpace those trying to develop internally.

2. Integrate Across Boundaries

Chinese engineers worked alongside Persian specialists and Muslim technicians. The Mongols didn’t care about origin – only capability.

Modern parallel: Diverse teams combining different traditions often produce breakthrough solutions.

3. Institutionalize Knowledge

Lessons from each siege were captured and spread across the entire Mongol military. Individual learning became organizational capability.

Modern parallel: Knowledge management systems that capture and distribute learning are force multipliers.

4. Combine Hard and Soft Power

Siege technology was only half the system. Terror, reputation, and the surrender calculus were equally important.

Modern parallel: Technical capability without strategic communication and incentive design is incomplete.

Conclusion: The Nomads Who Took the Cities

The Mongol siege revolution is a story of radical adaptation. A people whose entire civilization was built on mobility learned to destroy the fortifications that had stopped every previous steppe empire.

They did it not through genius invention, but through:

• Ruthless acquisition of expertise
• Systematic learning and improvement
• Integration of captured knowledge
• Combination with psychological warfare

Within a generation, the Mongols went from nomads who had never seen a siege engine to operators of the most effective siege force in history. No walls could stop them. No city was safe.

The lesson echoes across centuries: when organizations commit to learning – truly commit, with resources and systemic support – they can acquire capabilities that seem impossibly distant from their starting point.

The Mongols started with horses and bows. They ended up masters of walls, towers, and catapults.

What capability does your organization think is too far from its core? The Mongols would suggest that’s merely a failure of imagination – and acquisition strategy.

This post is part of the Mongol Empire series, exploring the military, economic, and organizational innovations that built history’s largest contiguous empire.

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