The Kuromaru (黒丸), or Black Fortress, was a fortress of Kanrei Shiba Takatsune located in Japan's Echizen Province (present-day Nittazuka, Fukui, Fukui Prefecture). It was attacked twice during the Nanboku-chō Wars of the 14th century, during which it was likely both built and destroyed.

The fortress was first attacked in August 1338 by a small force under Nitta Yoshisada, numbering roughly fifty horse, in what would come to be known as the battle of Fujishima. Hosokawa Akiuji had been ordered by Ashikaga Takauji to aid Shiba Takatsune in the defense, and encountered Nitta's force a short distance from the fortress.

The warrior monks of the Heisen-ji monastery, originally part of Nitta's force, were bribed by Shiba to abandon the attack, and Nitta's rush to fill the ensuing gap in his formations led to him and his horsemen running into Hosokawa's.

In the battle which followed, Nitta was mortally wounded by an arrow.

The following year, upon his enthronement, Emperor Go-Murakami ordered the late Nitta's brother, Wakiya Yoshisuke, to lead another attack on the fortress. This assault was successful, and ended in Shiba's surrender.

Chapter 20 of the Taiheiki chronicles the events of the death of Yoshisada at Kuromaru. It states that an arrow wounded his horse, making it unable to jump over a ditch. It fell and trapped Yoshisada's left leg. An arrow in the flurry struck him between the brows, and he drew his sword and slit his throat. The resemblance to the death of Minamoto no Yoshinaka as described in the Heike Monogatari has led to questions about whether the account is factual.

In 1656, a farmer tilling the land near the site of the battle uncovered a helmet and presented it to Matsudaira Mitsumichi, daimyo of the Fukui Domain. The construction of the helmet indicated that it had belonged to a high-ranking warrior, and the domain judged that it had belonged to Yoshisada. The helmet is now at the Fujishima Shrine in Fukui [1]. A marker, the Nittazuka, stands at the site where the farmer found the helmet.

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