Battle of Guju Fortress 1231: Mongol Siege and Outcome

In 1231, Bak Seo, Kim Gyeong-son, and others at Guju Fortress resisted a Mongol siege. They prevented its capture by assigning defensive sectors to different units and repairing the walls. Although they surrendered the following year under royal orders, their defense is remembered for withstanding repeated siege attacks.

This article follows the biography of Bak Seo in Goryeosa, published on Our History Net, and was checked in November 2026.

The Difference Between the Battle of Guju Fortress and the Great Victory at Gwiju

The Battle of Guju Fortress was a defensive battle against Mongol forces in 1231. On Our History Net, it can also be found under the name Battle of Gwiju Fortress. It is separate from the Great Victory at Gwiju in 1019, when Gang Gam-chan defeated the Khitan army.

The distinction between these events is based on “Bak Seo” on Our History Net and the Great Victory at Gwiju chronology in the Korean History Database. Our History Net, “Bak Seo”, Korean History Database, Great Victory at Gwiju Chronology.

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Guju Fortress resisted capture but submitted in 1232 by royal order after the court agreed to peace.AI-generated image

The Mongol Invasion and the Troops Gathered at Guju Fortress

Troops gathered at Guju Fortress as Mongol forces advanced south. The Mongols invaded Goryeo in the 8th month of 1231. They cited the killing of the Mongol envoy Jeogoyeo in 1225 as their justification for the invasion.

“The Mongol Invasions of Goryeo” on Our History Net also explains the broader background. It presents an interpretation linking the invasion of Goryeo to Mongol preparations to conquer the Jin dynasty. The envoy’s killing alone therefore cannot fully explain the causes of the invasion. Our History Net, “The Mongol Invasions of Goryeo”.

Guju was an inland military stronghold in northwestern Goryeo. Bak Seo, the military commander of the Northwestern Region, established his defensive headquarters there. Kim Gyeong-son and others who had withdrawn from Jeongju joined troops from nearby areas.

This deployment assigned troops from several regions to different defensive sectors. It is one reason the defense of Guju Fortress cannot readily be described as the achievement of a single commander. The gathering and deployment of troops are explained in New History of Korea. New History of Korea, “Victories at Gwiju Fortress and Jaju Fortress”.

A Battle Chronology from 1231 to 1232

Fighting at Guju Fortress recurred from autumn through winter in 1231. The surrender under royal orders is described as taking place in the first month of the following year. The months below follow the lunar calendar used in the records.

This month-by-month chronology was compiled by comparing the accounts in New History of Korea and Our History Net. New History of Korea, “Victories at Gwiju Fortress and Jaju Fortress”, Our History Net, “Bak Seo”.

Responses to Encirclement and Siege Attacks

Goryeo’s defenders adjusted their defenses to the Mongols’ methods of attack. Siege warfare means fighting to attack and capture a fortress. The biography of Bak Seo in Goryeosa describes attacks ranging from assaults on the gates to attacks using ladders.

Here, pocha refers to stone-throwing siege equipment. The name in the record cannot simply be equated with modern gunpowder cannons. Daewupo, too, is described in this record as a weapon fitted with a large blade. Original Text of the Biography of Bak Seo in Goryeosa and Commentary from Our History Net.

How Wall Repairs and Royal Orders Changed the Course of the Battle

The key to sustaining the defense was immediately restoring collapsed sections of wall. Goryeosa describes repairs made whenever the walls were damaged. It also records that iron cables were used to reinforce them.

Damage to the walls did not immediately lead to the fall of the entire fortress. The defenders repaired damaged sections and prepared for the next attack. New History of Korea highlights the residents’ role in this repair work. New History of Korea, “Victories at Gwiju Fortress and Jaju Fortress”.

The turning point that ended the fighting was a royal order sent from the court. Bak Seo had earlier rejected an appeal to surrender conveyed in a document from Wang Jeong, Duke of Hoean. The following year, Choe Rim-su and Min Hui delivered the royal order.

We have already sent Jeong, Duke of Hoean, to make peace with the Mongol army, and our three armies have also surrendered. Stop fighting, come out, and surrender.

Goryeosa, volume 103, “Biographies” 16, “Bak Seo”: Choe Rim-su and Min Hui’s appeal to surrender.

After repeated persuasion, Bak Seo surrendered under royal orders. The immediate reason the fighting ended must therefore be distinguished from the fortress falling through military action. These circumstances are recorded in the biography of Bak Seo in Goryeosa. Our History Net, “Bak Seo’s Battle at Gwiju Fortress”.

The Battle’s Outcome and the Continuing Mongol Invasions

Guju Fortress avoided capture, but its defense did not reverse the course of the war across Goryeo. Goryeo’s main army was defeated at the Battle of Anbuk-bu. When Mongol forces advanced toward Gaegyeong, the court sought peace.

Here, ganghwa means an agreement to stop fighting. It is a different term from the Ganghwa in the relocation of the capital to Ganghwa Island that same year. Goryeo moved its capital to Ganghwa Island in 1232. A second Mongol invasion followed that year. Our History Net, “The Mongol Invasions of Goryeo”.

How to Read the Numbers in the Records

Numbers relating to the Battle of Guju Fortress must be read with attention to what they count. Treating the size of a particular unit as the entire fortress garrison changes the meaning. The duration of one round of fighting must also be distinguished from that of the battle as a whole.

These figures appear in the original text of Bak Seo’s biography in Goryeosa. The record also goes on to describe renewed attacks after the initial fighting. The beginning and end of the Battle of Guju Fortress therefore cannot be defined by the 30 days alone. Our History Net, Original Text of “Bak Seo’s Battle at Gwiju Fortress”.

Records and Interpretations

What Is Directly Recorded

The main historical source is Goryeosa, volume 103, “Biographies” 16, “Bak Seo.” It covers troop deployments, responses to siege attacks, wall repairs, and surrender under royal orders. It also describes the circumstances of Bak Seo’s retirement to his hometown after the battle.

It also recounts a veteran Mongol commander’s praise for the defense. This should be read as an assessment included in Goryeosa. That statement alone cannot establish how this battle ranks against battles from other periods. Our History Net, “Bak Seo’s Battle at Gwiju Fortress”.

Assessments Shared by the Historical Commentaries

The commentaries consulted recognize the successful defense of Guju Fortress. New History of Korea discusses the participation of both regional troops and residents. “Bak Seo” on Our History Net distinguishes the military achievement from the subsequent surrender.

Reading these accounts together also reveals the roles of people other than the commanders. Troops fighting in their assigned sectors and residents repairing the walls supported the defense. This explanation draws together the commentaries consulted; it is not a single view shared by all historians. New History of Korea, “Victories at Gwiju Fortress and Jaju Fortress”, Our History Net, “Bak Seo”.

What Remains Uncertain

The commentaries differ in their accounts of Bak Seo’s activities after the war. New History of Korea states that he was later reappointed to office. “Bak Seo” on Our History Net considers the timing of his appointment as munha pyeongjangsa uncertain. It also leaves unresolved whether the appointment was made during his lifetime or awarded posthumously.

It is therefore difficult to turn Bak Seo’s later life into a detailed chronology of his return to office. The records consulted also do not establish the total troop strength on either side. Nor is there any basis for adding accounts of Bak Seo’s feelings or speeches of encouragement absent from the records. Our History Net, “Bak Seo”.

Why This Battle Is Studied as a Distinct Event Today

The Battle of Guju Fortress shows both local defense and national decisions about war. The successful defense of the fortress and the court’s decision to make peace were events at different levels. Both must be considered to understand the circumstances behind the eventual surrender.

The records of wall repairs also draw attention to the work that sustained the fighting. This perspective considers the commanders’ actions alongside the residents’ roles. These are details lost when military history is reduced to victories, defeats, and the names of individuals.

Questions checked against the record

Did Goryeo Win the Battle of Guju Fortress?

The defense succeeded in preventing the fortress from being captured. However, the defenders later surrendered under royal orders. The achievements of the local defense must be distinguished from the political conclusion.

Did Guju Fortress Refuse to Surrender to the End?

It did not give in to Mongol attacks or the initial appeals to surrender. However, it surrendered under royal orders in 1232. Any statement that it did not surrender needs to distinguish between these points in time.

Did the Battle Last Only 30 Days?

The 30 days refer to one round of siege fighting described in the record. The Mongols attacked again afterward. The full course of events must be considered from autumn 1231 to the first lunar month of 1232.

What Roles Did Kim Gyeong-son and Bak Seo Play?

Bak Seo commanded the troops gathered at Guju Fortress. Kim Gyeong-son’s unit was responsible for the southern side. Kim Jung-on’s unit defended the eastern and western sides.

Who Delivered the Royal Order?

Choe Rim-su and Min Hui appear in the biography of Bak Seo in Goryeosa. They conveyed news of the court’s peace agreement and the surrender of the three armies. After repeated persuasion, Bak Seo obeyed the royal order.

Where Can the Original Text and Translation Be Found?

They can be found in “Bak Seo’s Battle at Gwiju Fortress” on Our History Net. The page includes the original text of Bak Seo’s biography from Goryeosa. A translation and separate commentary are also provided.