The 'Capital of the Sea' in the Sengoku Period: Miyoshi Nagayoshi's Iimoriyama Castle and the Secret of the Phantom Giant Lake 'Fukono-ike'

When people think of the capital during the Sengoku period, many likely think of Kyoto.

However, shortly before Oda Nobunaga entered Kyoto, the center of Japan's politics, economy, and military in the mid-16th century was located at Iimoriyama Castle, a mountain castle 314 meters above sea level that towers over the present-day cities of Daito and Shijonawate in Osaka Prefecture.

Why did the hegemon of the Kinai region, Miyoshi Nagayoshi, choose this mountain castle as his residence instead of Kyoto?

The answer lay in the 'world of water' that spread out before his eyes.

In this article, we will explore the secrets of the magnificent water network woven by Osaka Bay during the height of Nagayoshi's power and the giant lake 'Fukono-ike,' which has now vanished from the map.

1. The giant lagoon 'Fukono-ike' that spread at the foot of Iimoriyama Castle

While the area around Iimoriyama Castle is now urbanized, during the Sengoku period, a vast body of water called 'Fukono-ike' spread out at the western foot of the mountain.

It was a massive lake and marshland stretching approximately 8 km from north to south and 4 km from east to west.

However, this Fukono-ike was not a deep blue lake.

Sediment flowing in from the Yamato and Yodo rivers had accumulated, and the water depth was only about 3 to 5 shaku (approximately 0.9 to 1.5 meters).

It was a vast 'lagoon' overgrown with reeds, with a depth only reaching an adult's waist to chest.

This shallow, giant body of water was the greatest weapon that made Iimoriyama Castle a 'capital'.

Fukono-ike was directly connected to the Yodo River and Osaka Bay through the Yamato and Neya river systems, functioning as a massive logistics highway connecting the sea and the rivers.

2. The vitality born from 'transshipment' and the water city loved by missionaries

Large, deep-draft ships that sailed Osaka Bay (such as Sekibune or Atakebune) could not directly enter Fukono-ike, which had a water depth of only around 1 meter.

Therefore, goods transported from Awa (Tokushima Prefecture), Awaji Island, or overseas were transferred to flat-bottomed, shallow-draft river boats (such as Takasebune) near the mouth of the Yodo River.

Countless small boats wove through the reed-filled waterways, carrying goods to the foot of Iimoriyama Castle.

Also, because the entirety of Fukono-ike was shallow, several small islands would appear when the water level dropped.

Built on the largest of these islands was 'Sanga Castle,' the residence of Miyoshi's senior retainer, Sanga Yoriteru.

Surrounded on all sides by shallows and mudflats, it was truly an impregnable water fortress.

Because Yoriteru received Christian baptism, a magnificent cathedral was built on this island.

Records from missionaries such as Luís Fróis describe how, at Easter, neighboring Christians would gather at the church in small boats, suggesting a landscape that resembled a water city like Venice.

3. The Control Tower Governing the 'Osaka Bay Economic Zone'

The Osaka Bay coast, located down the waterway from Fukano-ike, also boasted unprecedented prosperity under the Miyoshi administration.

Control of the International Trade Ports of 'Sakai' and 'Hyogo-tsu'

Nagayoshi seized control of the economic power of Sakai, a trade hub with Ming China and the Southern Barbarians (Europeans).
He acquired large quantities of state-of-the-art weapons like matchlock guns, raw materials for gunpowder (saltpeter), and high-quality ceramics, solidifying the foundation of his administration.

The Strongest Naval Police: The 'Atagi Navy'

The one who held command of the seas in Osaka Bay was the 'Atagi (Awaji) Navy,' led by Nagayoshi's younger brother, Atagi Fuyuyasu.
By keeping watch from the Kitan Strait to the Akashi Strait, they prevented enemy naval blockades and enabled the rapid transport of troops and supplies from Shikoku.

Tense Relations with Ishiyama Hongan-ji

Near the mouth of the Yodo River (present-day Osaka Castle) sat 'Ishiyama Hongan-ji,' a religious city with its own independent economic zone and military power.
Nagayoshi's gaze was always fixed on the movements of this massive power as well.

Conclusion: The 'City of the Sea' Nagayoshi Dreamed Of

Looking at it from this bird's-eye view, it becomes clear that during the peak of Miyoshi Nagayoshi's power, the entire Osaka Bay area formed a massive 'Osaka Bay Economic Zone' that accumulated logistics, wealth, information, and the latest international culture, including Christianity.

Iimoriyama Castle was not merely a defensive stronghold with a good view.

It was an extremely rational 'command tower' designed to oversee and control the entire magnificent water network stretching from Awa to Awaji Island, Osaka Bay, the Yodo River, and Fukano-ike.

Miyoshi Nagayoshi, who ruled the sea while climbing the mountains.

When I imagine the scenery he was looking at from the summit of Iimoriyama Castle, the grand scale of the Sengoku period emerges once again.

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