Complete Guide to the History and Highlights of Iwakura Castle | A Famous Castle That Became a Turning Point in Oda Nobunaga’s Unification of Owari
What is Iwakura Castle?
Iwakura Castle (Iwakura-jo) was a flatland castle located in Shimohonmachi, Iwakura City, Aichi Prefecture, and is now preserved as a designated historical site of Iwakura City. As the base of the Oda Ise-no-kami family (Iwakura Oda clan), who ruled Owari Province divided into north and south, it was an important strategic point in Owari during the Sengoku period, alongside Kiyosu Castle.
Built on a natural levee approximately 10 meters above sea level on the right bank of the Gojo River, this castle became the greatest obstacle to Oda Nobunaga’s unification of Owari. The battles fought from 1558 to 1559 marked a crucial turning point in Nobunaga’s path to unifying Japan. It is also known as the birthplace of Yamauchi Kazutoyo, who later became the lord of Tosa Domain.
Construction and Historical Background of Iwakura Castle
Construction History
There are various theories regarding the construction period of Iwakura Castle, but the most widely accepted theory is that it was built by Oda Toshihiro around Bunmei 11 (1479). However, there are also records suggesting that it was built by Oda Nobuyasu during the Meio era (1492-1501).
Oda Toshihiro served as the deputy governor of Owari Province, with Shimotsu Castle as his base. However, during the chaotic period following the Onin War (1467-1477), his conflict with Oda Toshisada, who was appointed deputy governor of Owari by the shogunate, intensified. Because Shimotsu Castle was devastated during this conflict, it is believed that he built Iwakura Castle as a new base after peace was achieved.
The Two Major Powers of Owari Province
The Oda Ise-no-kami family, based at Iwakura Castle, controlled the four upper districts of Owari Province (Niwa, Haguri, Nakashima, and Kasugai). Meanwhile, the Oda Yamato-no-kami family (Kiyosu Oda clan), based at Kiyosu Castle, controlled the lower four districts (Aichi, Chita, Kaito, and Kaisei districts), effectively dividing Owari Province into north and south.
During the Ōnin War, the conflict between the two families became decisive when Oda Toshihiro sided with the Eastern Army and Oda Toshisada with the Western Army, leading to a power struggle within Owari for several decades. This division became the backdrop for the later need for Oda Nobunaga to unify Owari.
Conflict with Oda Nobunaga and the Battle of Ukino
The Rise of Nobunaga and the Iwakura Oda Clan
In 1534, when Oda Nobuhide, Nobunaga’s father, seized control of Kiyosu Castle, the Kiyosu Oda family’s power rapidly expanded. Backed by economic and military strength, Nobuhide effectively controlled southern Owari, and tensions with the Iwakura Oda clan escalated. When Nobuhide died in 1552, the young Oda Nobunaga inherited the family headship. At that time, the lords of Iwakura Castle were Oda Nobuyasu and his son, Oda Nobukata. Immediately after inheriting the family headship, Nobunaga was preoccupied with controlling his retainers and fighting against neighboring forces, but he gradually made his ambition to unify Owari Province clear.
The Battle of Ukino in 1558
In 1558, the all-out confrontation between Oda Nobunaga and the Iwakura Oda clan finally began. Nobunaga advanced his army with the aim of capturing Iwakura Castle, and clashed with the forces of Oda Nobuyasu and his son Nobukata at Ukino (near present-day Ichinomiya City).
This Battle of Ukino was a fierce battle, with both sides suffering numerous casualties, but ultimately the Iwakura forces were routed and fled into the castle. Nobunaga’s army then besieged Iwakura Castle, transitioning into a long siege.
The Fall of Iwakura Castle
In 1559 (Eiroku 2), after a siege lasting approximately one year, Iwakura Castle finally fell. Oda Nobuyasu and his son Nobukata surrendered, and the four northern districts of Owari Province came under Nobunaga’s control. With the capture of Iwakura Castle, Oda Nobunaga achieved his long-held ambition of unifying Owari Province, taking a firm first step towards conquering Japan.
During the castle’s fall, Yamauchi Moritoyo (father of Yamauchi Kazutoyo), who fought as a general on the Iwakura side, was killed in battle, sharing the castle’s fate. Kazutoyo was 14 years old at the time. After his father’s death, he became a wanderer, but later served Toyotomi Hideyoshi and Tokugawa Ieyasu, rising to become the first lord of the Tosa Domain.
The Relationship Between Yamauchi Kazutoyo and Iwakura Castle
The Birthplace of Kazutoyo
Yamauchi Kazutoyo is said to have been born in Iwakura in July of Tenbun 14 (1545), the son of Yamauchi Moritoyo. Moritoyo was a samurai serving the Iwakura Oda clan, and Kazutoyo spent his childhood in the town of Iwakura Castle.
When Iwakura Castle fell in Eiroku 2 (1559), his father, Moritoyo, fought against Oda Nobunaga’s army as a samurai on the Iwakura side and was killed in battle. At this time, Kazutoyo was 14 years old, and he escaped the castle with his mother and siblings, and thereafter he endured a period of hardship, moving from place to place.
A Story of Rise to Power
After his father’s death, Kazutoyo became a retainer of Oda Nobunaga, and later served Toyotomi Hideyoshi, where he rose to prominence. During the Battle of Sekigahara, he sided with Tokugawa Ieyasu, and for his service, he was granted the entire province of Tosa, worth over 200,000 koku, becoming the first lord of the Tosa Domain.
The life of Kazutoyo, the son of a loser at the fall of Iwakura Castle, who eventually rose to become a daimyo, is recounted to posterity as a rags-to-riches story of the Sengoku period. Even today, Iwakura City honors Yamauchi Kazutoyo as a “local hero” and continues to promote his achievements.
Structure and Scale of Iwakura Castle
Location and Layout of the Castle
Iwakura Castle is a flatland castle built on a natural levee on the right bank of the Gojo River, at an elevation of approximately 10 meters. It was a strategically important location surrounded by rivers and wetlands, and its location also considered the advantages of water transport.
Regarding the size of the castle, it is believed to have had a structure with the main keep at the center, surrounded by the second and third baileys. Archaeological excavations have confirmed that the Hagiwara-Taki Prefectural Road runs east-west across the northern part of the main keep site, indicating that the castle grounds extended over a fairly wide area.
Remains and Archaeological Excavations
Currently, there are almost no clearly identifiable remains visible on the surface of the Iwakura Castle site. However, archaeological excavations conducted in conjunction with the construction of the prefectural road have uncovered Yayoi period remains in the lower layers of the castle site, along with castle-related remains, revealing that this area was a place of human habitation from ancient times.
The investigations have confirmed traces of moats, earthworks, and building foundations, gradually revealing the true nature of Iwakura Castle as a medieval castle. Excavated artifacts provide insights into life and battles within the castle at that time.
Defensive Structures
Iwakura Castle, a flatland castle, possessed a defensive system that skillfully utilized the natural terrain. The flow of the Gojo River was used as an outer moat, and the surrounding wetlands served as a natural barrier. Furthermore, it is presumed that multiple baileys were protected by artificially dug moats and earthworks.
The fact that the siege of Iwakura Castle in 1559 lasted for approximately one year demonstrates its high level of defense. The fact that it was able to withstand Oda Nobunaga’s powerful army for such a long period is evidence that these defensive structures functioned effectively.
The Current State of Iwakura Castle Ruins
Preservation as a Historic Site
The current Iwakura Castle ruins are located in the area of Shimohonmachi, Iwakura City, and are protected as a city-designated historical site. Stone monuments marking “Iwakura Castle Ruins” and “Oda Ise-no-kami Castle Ruins” stand at the site of the main bailey, allowing visitors to imagine the castle’s former appearance.
While the area surrounding the castle ruins is now residential and farmland, making it difficult to imagine the castle’s former form, local efforts continue to carefully preserve it as a historical heritage site. Information boards have also been installed to convey the history of Iwakura Castle to visitors.
Monuments and Commemorative Activities
A monument marking the birthplace of Yamauchi Kazutoyo has been erected at the Iwakura Castle ruins. Every year, Iwakura City holds events and lectures related to Kazutoyo, actively working to pass on local history to future generations.
Furthermore, as a site marking a crucial turning point in Japanese history—Oda Nobunaga’s unification of Owari Province—it attracts many history enthusiasts and fans castle, and is gaining attention as a local historical tourism resource.
Access Information
Access by Public Transportation
The Iwakura Castle ruins are approximately a 15-minute walk from Iwakura Station on the Meitetsu Inuyama Line. Head northeast from the station, cross the Gojo River, and the ruin castles are located in a residential area. There are guide signs around the station, so you should be able to arrive relatively easily.
If using the JR Tokaido Main Line, it is convenient to transfer to the Meitetsu line at Nagoya Station and head to Iwakura Station. The journey from Nagoya Station to Iwakura Station takes approximately 20 minutes.
Access by Car
By car, the site is approximately 15 minutes from the Komaki-Minami Interchange on the Nagoya Expressway, or approximately 20 minutes from the Ichinomiya Interchange. While there is no dedicated parking lot near the castle ruins, you can use the parking lots of nearby public facilities.
Public facilities such as the Iwakura City Hall and the Iwakura City Lifelong Learning Center are relatively close, making it possible to visit the castle ruins on foot from these locations. However, as the site is located within a residential area, consideration for nearby residents is necessary when visiting.
Points to Note During Your Visit
Visiting the Iwakura Castle Ruins is free of charge; there is no entrance fee. However, since very few remains are left, learning about the history beforehand will allow for a deeper understanding.
The area around the castle ruins also features a row of cherry trees along the Gojo River, offering beautiful cherry blossoms in the spring. Enjoying the seasonal scenery along with a historical walk is highly recommended. The Iwakura City Historical Museum also holds exhibits related to Iwakura Castle and Yamauchi Kazutoyo, so visiting both will deepen your understanding.
Nearby Historical Spots
Kiyosu Castle
Kiyosu Castle (also known as Kiyosu-jo) is an essential site, considered a counterpart to Iwakura Castle. Known as the residence of Oda Nobunaga, its keep has been restored. Located about 30 minutes by car from Iwakura Castle, it’s an important spot for tracing the history of Owari’s unification.
Komakiyama Castle
Komakiyama Castle, built by Oda Nobunaga after unifying Owari, is also about 25 minutes by car from Iwakura Castle. It’s a valuable historical site for understanding the development of Nobunaga’s castle-building techniques, and the view from the mountaintop is magnificent.
Inuyama Castle
Inuyama Castle, designated a National Treasure, is located about 15 kilometers north of Iwakura Castle. It’s a valuable castle with an existing keep, preserving the appearance of a Sengoku period castle. As a strategic point protecting the border between Owari and Mino, it was a castle highly valued by Oda Nobunaga.
The Impact of Iwakura Castle on Japanese History
Oda Nobunaga’s First Step Towards Unifying Japan
The capture of Iwakura Castle was a definite first step for Oda Nobunaga toward unifying Japan. By achieving the unification of Owari Province, Nobunaga gained a strong economic base and military power, paving the way for his subsequent march to Kyoto and his quest for national unification.
If he had failed to capture Iwakura Castle, Nobunaga’s history could have been dramatically different. In that sense, the fall of Iwakura Castle in 1559 can be considered one of the major turning points in Japanese history.
Formation of the Owari Samurai Corps
After capturing Iwakura Castle, Nobunaga absorbed the former retainers of the Iwakura Oda clan, building a more powerful military organization. The Owari samurai corps formed during this process became the core of Nobunaga’s army, playing a significant role in battles such as the Battle of Okehazama and the conquest of Mino Province.
Place in the History of Castles in the Sengoku Period
Iwakura Castle holds an important position in the history of castles as a typical example of a flatland castle from the transitional period between the medieval and early modern periods. Its defensive system, utilizing natural terrain, and its location, which considered river transportation, clearly reflect the castle-building philosophy of the time.
Summary
Iwakura Castle was a castle that marked a crucial turning point in Japanese history: Oda Nobunaga’s unification of Owari Province. As the base of the Oda Ise-no-kami family, it controlled the four northern districts of Owari and was one of the two major strongholds of Owari, alongside Kiyosu Castle.
The battles against Oda Nobunaga from 1558 to 1559 were the first major test on Nobunaga’s path to unifying Japan, and this victory laid the foundation for his later rapid advances. It is also known as the birthplace of Yamauchi Kazutoyo, and is considered the origin of many rags-to-riches stories in the Sengoku period.
Iwakura Castle was a castle that marked a significant turning point in Japanese history, particularly during the unification of Owari Province. Although no significant remains currently exist at the Iwakura Castle site, it is preserved as a designated historical site of Iwakura City and cherished as a valuable historical heritage of the region.
Its convenient location, within walking distance of Meitetsu Iwakura Station, makes it easily accessible as part of a historical tour around Nagoya. Visiting it along with nearby castles such as Kiyosu Castle, Komakiyama Castle, and Inuyama Castle will allow for a deeper understanding of Oda Nobunaga’s footsteps.
