＜特別編＞あの日をたどろう！モンゴル襲来の最前線・元寇防塁へ

The Sea the Mongols Crossed, Seen from Bishamon-yama

COTEN RADIO's first location recording began at Bishamon-yama in Imazu, Nishi Ward, Fukuoka. The hosts climbed a road inaccessible by car, taking about 20 minutes on foot to reach the summit temple Seigan-ji (the inner sanctuary), from which they looked out over Hakata Bay. Nokono-shima island was visible offshore, and through gaps in the coastal trees they could make out the ridgeline of the Genkō Bōrui. The reaction was that the spot felt too peaceful to associate with an invasion — yet this was the very sea where roughly 30,000 Mongol soldiers appeared during the Battle of Bun'ei.

The large Mongol vessels could not reach the shore because of Hakata Bay's shallow waters. They had to anchor offshore and transfer troops to smaller boats for a continuous shuttle to the beach, making a massed, simultaneous landing structurally impossible. That limitation, later combined with the stone wall, would hand a decisive advantage to the defenders. The Battle of Bun'ei is said to have involved some 900 ships.

The Stone Wall Half-Buried in the Sand

Descending the hill, the hosts walked to the coastal ruins of the Genkō Bōrui. At its original height of 2–3 m, only the upper section now breaks the surface — the rest lies beneath 600 years of accumulated sand. One host relayed what Fukuoka City's on-site official had explained in advance: "Back when the ground was lower, this wall would have looked far taller."

In the Imazu section alone the wall extends about 3 km; along the full length of Hakata Bay it ran approximately 20 km. With special permission from Fukuoka City, the hosts entered sections normally closed to the public and inspected the cross-section. The wall was both wide enough and tall enough that a horse could not jump it, and its design allowed samurai standing on top to shoot arrows down at enemies attacking from the sea. The official noted that the wall effectively stopped the Mongol army from making any substantial landing.

How the Stones Reveal Each Clan's Character

Construction was divided among Kyushu warrior clans by section. The on-site signboards recorded the following assignments: Imazu to clans from Ōsumi and Hyūga, the Hakozaki area to Satsuma, Iki-no-Matsubara to Higo, and Akashi-no-Hama to Hizen. The wall alternates between dressed stone and basalt. A separate excavation about ten years ago at a former campus site in Hakozaki also uncovered part of the wall.

Walking the roughly 3 km stretch made the divisions tangible: at the boundaries between sections the size and style of the stonework changes noticeably. Bold stretches of large, rough-hewn boulders sit right next to careful courses of small, neatly laid stones. The hosts laughed that it was like seeing the difference in a director's sensibility, but settled on the conclusion that it reflects the scale and resources of each warrior group. Sections with shorter runs and finer stones were likely assigned to less wealthy clans. Notably, the builders received no reward for the work; it was ordered as service owed under the existing system of obligations to the shogunate.

The Great Wall It Is Not — and Why That Matters

The wall is nowhere near the Great Wall of China in finish. Heights are uneven, the line is far from straight, and the stone types look random. The hosts read this as evidence of an emergency construction project where function mattered and appearance did not. Compared with the sophisticated stone castle walls that would appear in the Sengoku period and after, it is clearly rough work.

And yet the sheer scale was assembled in a short time, and the reason lies in the acute sense of danger felt by Kyushu's warrior clans. Central power in Kamakura and Kyoto had no firsthand experience of the invasion, but the Kyushu warriors who had been trampled by Mongol cavalry in the Battle of Bun'ei knew exactly what was at stake. For them, building the wall ahead of the Battle of Kōan was not a political obligation — it was a matter of survival. "The ones with the strongest sense of crisis were the Kyushu warriors," one host said, and on that note the field visit came to a close.