The siege of Kasagi (3-31 October 1331) saw the force of the Kamakura Shogunate capture the Emperor Go-Daigo's refuge at Kasagi, a success that for a time appeared to have crushed the Imperial cause (Genko War, 1331-33).
Towards the end of September 1331 it was becoming clear that the Shogunate was aware that the Emperor Go-Daigo had been plotting to overthrow them and restore direct Imperial rule of Japan. The Shogunate sent an army to Kyoto to arrest the emperor and force him into exile. Go-Daigo was warned just in time, and escaped from Kyoto. First he went to Nara, the southern capital, but he wasn't sure of his support there and he moved on, eventually reaching Kasagi, a strong castle built on top of a 600ft hill overlooking the Kizu River.
Go-Diago reached Kasagi on the 27th day of the eighth month of 1331 (29 September 1331). At first nobody was willing to join him, but after news spread of the defeat of the Bakufu at Karasaki Beach (near Mount Hiei) the monks of Kasagi decided to support his cause. He also received supports from loca warriors, but only from minor lords. According to our main source for this period, the Taiheiki, none of his new supports commanded one hundred men or more. He did gain the support of Kusunoki Masashige, one of his most able followers, during this period.
The Bakufu responded to the Imperial revolt and the setback at Karasaki quickly and in large numbers. The Taiheiki claims that 100,000 riders arrived on the first day of the ninth month (3 October 1331). The Bakufu leaders decided to attack Kasagi on the next day (4 October 1331, the second day of the ninth month).
The battle began a day early, on 3 October. Two Bakufu leaders, Takahashi Matashiro and Kobayakawa both decided to attack early, possibly in an attempt to win glory for themselves. According to the Taiheiki the defenders had 3,000 cavalry, and used them to rout Takahashi at the Kozu River. Kobayakawa was following further behind, and was also defeated.
Effective command of the Bakufu army was held by the examiners Kasuya Saburo Muneaki and Suda Juro Saemon, the second-ranking officials in the Rokuhara (the Shogunate's base in Kyoto). They realised that this setback, following so soon after the defeat at Karasaki, could encourage support for the Emperor. They decided to launch a massive assault on Kasagi on the third day of the ninth month (5 October 1331). Again according to the Taiheiki 7,600 horsemen were to attack from the south, 25,000 from the east, 12,000 from the north and 32,000 from the west, a total of over 75,000 men.
Kasagi was a very strong defensive position. The top of the mountain was fortified, as was the long and difficult approach road. Soon after dawn on 5 October the Bakufu forces fired signal arrows and then advanced towards the castle. The defenders remained hidden behind their walls until the Bakufu forces were close to the castle walls, and only then revealed themselves. According to the Taiheiki the castle was defended by 3,000 armoured warriors and an unstated number of archers.
After a ritual exchange of challenges and a demonstration of archery from within the castle an assault began. The attackers reached the castle gate, but were unable to make any more progress and were forced to pull back. The attacking army pulled back to a safe range and kept up a long range harassing attack on the castle.
The besieging army was disturbed by news of other outbreaks of revolt. Kusunoki Masashige, a minor warrior who would later become one of the Emperor's key supporters, had rebelled and was defending his castle of Akasaka, while Sakurayama Shirō rebelled in Bingo Province, on the Honshu coast of the Inland Sea. This news convinced the Bakufu to send reinforcements towards Kyoto.
Again according to the Taiheiki it was the approach of this army that triggered the final assault on Kasagi. Two newly arrived members of the Bakufu army, Suyama Tozo Yoshitaka and Komiyama Jiro, who between them led fifty kinsmen and retainers, decided to launch a night-time attack on the castle in an attempt to win fame before the arrival of the larger army made that impossible.
This small party decided to attack on the last day of the ninth month (31 October 1331). This was a dark moonless night, and the attackers were able to climb the cliffs below the north wall of the castle without being detected. Once in the castle they discovered that the garrison was guarding the more vulnerable west, south and east walls but had left the north wall almost unwatched. They were able to sneak around the castle and learn its layout and even found the Emperor's residence in the main hall.
Suyama and his party then made their way to a high peak above the main hall, lit a beacon fire in a deserted compound and raised a battle cry. The fire and the noise were heard outside the castle. The rest of the attacking army had no idea that any of their men were inside the walls, and instead assumed that some of the garrison had decided to change sides. The besieging soldiers made their own war cries, making a noise that convinced the defenders that a large army had somehow broken into the castle. The morale of the defending troops broke, and they attempted to flee from the besieged castle (this alone suggests that the attacking army was much smaller than the 70,000-75,000 men given by the Taiheiki, for an army of that size would have been able to impose far too close a blockade to allow anyone to attempt to escape).
While the garrison attempted to escape Suyama and his men ran around the castle, lighting fires and causing chaos. As smoke spread throughout the castle Go-Daigo joined the flight, and managed to escape into the night. His aim was to reach Akasaka, which was still being held for him, but after several days wandering across the countryside he was captured by forces loyal to the Bakufu and taken to Kyoto. Although Go-Daigo refused to abdicate, the Bakufu soon enthroned crown prince Kazuhito as Emperor.
In the spring of 1332 Go-Daigo was exiled to the island of Oki and his cause looked to be doomed. Fortunately for him not all of his supporters had been captured. Prince Morinaga was still free and Kusunoki Masashige escaped at the end of the siege of Akasaka (1331) and fought on, leading a resistance movement that would eventually topple the Shogunate.