It is part of a larger power struggle between Takeda Shingen and Uesugi Kenshin.

[1] Some years before, Nagano Narimasa, lord of Minowa castle and faithful retainer to the Uesugi, died.

In order to protect the region from the depredations of the Takeda, the Nagano family kept his death a secret until his heir could settle into power.

The Takeda attacked in 1566, and were held off for a time, with the young heir, Nagano Narimori, and famous swordsman Kamiizumi Hidetsuna, leading the defense.

Intense hand-to-hand fighting eventually led to Hidetsuna leading a bold charge from the castle, which was initially successful.