Fortresses Under Siege: Tactics & Lessons

The Sengoku period, known as the ‘Warring States period,’ was a time of nearly constant civil war and political upheaval in 15th and 16th century Japan. Castles were not only strongholds of military power but also centres of governance and strategic control. Attacking and defending these fortresses required extensive planning, knowledge, and execution, as outlined by the Natori-Ryu school of military strategy.

The Source of Castle Warfare Tactics

Natori-Ryu was founded by Masazumi Natori in the 17th century, drawing from the knowledge accumulated by the Natori family while serving Takeda Shingen. This school focused on gungaku (military strategy), with extensive writings on castle construction, siege warfare, and shinobi tactics. Among its many teachings, special emphasis was placed on the role of spies and infiltrators, whose presence could turn the tide of battle.

Best Practices for Attacking and Defending a Castle

When Attacking a Stronghold:

• A well-built castle will not fall easily. Instead of direct assaults, employ deception, sabotage, and psychological warfare.

• Weak castles should be attacked with surprise tactics, striking before defenders can prepare.

• Avoid fully surrounding a besieged castle except at the beginning. Leaving an escape route encourages defenders to flee, where they can be captured and interrogated for intelligence.

• Use fire and flood tactics to weaken defenses and force surrender.

• If enemy forces have ample supplies, attack swiftly. If their resources are dwindling, take your time and let attrition do the work.

When Defending a Castle:

• Destroy bridges, burn excess firewood, and reinforce entrances to limit the enemy’s access and resources.

• Defenders must remain vigilant against internal betrayal. Regular troop rotations prevent conspiracies from forming within the ranks.

• Secure wells with lockable lids and station guards nearby to prevent enemy agents from poisoning the water supply.

• Use password-protected entry combined with a specific body movement for authentication, preventing enemy agents from infiltrating disguised as returning allies.

The Role of Spies in Castle Warfare

One of the most critical aspects of castle warfare was the presence of spies—both within the enemy ranks and among one’s own forces. Natori-Ryu warns against infiltrators who may:

• Feed false information to the defenders.

• Poison supplies and water sources.

• Use bribery and seduction to influence key figures.

• Act as signalers, marking weak points for an attacking force.

To counteract this, constant surveillance and strict control measures were necessary. The school advises using counterintelligence strategies, deception, and frequent loyalty tests to root out potential traitors.

Case Studies: When Castle Tactics Were Used or Ignored

Several historical sieges demonstrate the effectiveness—or failure—of these principles:

• Siege of Fushimi Castle (1600): A well-fortified stronghold, but ultimately fell due to betrayal. The besieging forces used hostages to coerce a defender into setting fire to the keep from within.

• Siege of Futamata: Takeda Katsuyori’s forces successfully identified a critical weakness—the enemy’s reliance on a water well—and used flood tactics to deprive defenders of their supply, forcing surrender.

• Siege of Yoshida Castle (1575): A defensive success where the defenders refused to be lured into open battle, causing the attackers to abandon their siege and seek easier targets.

• Siege of Noda Castle (1573): Enemy sappers drained the castle’s moat, eliminating its primary defense and forcing surrender.

Conclusion

Natori-Ryu’s teachings provide invaluable insight into the complexities of castle warfare, particularly in the realm of counterintelligence. While strong fortifications and well-armed soldiers were essential, the most dangerous threats often came from within. As history has shown, the presence of spies could determine the fate of an entire stronghold. Fortunately, new discoveries of historical manuscripts continue to shed light on these ancient strategies, ensuring that the art of ninjutsu and samurai warfare remains a subject of deep study and fascination.