When Nobutomo came of age, he entered into the service of Takeda Shingen, patriarch of the clan and lord of Kai province,[2] in the mountainous area of central Japan.

As Takeda Shingen drove south and west, Nobutomo would descend from the north, cutting off an escape route and blocking reinforcements.

When Toyama Kagetou, lord of Iwamura Castle, died of a sudden illness, the morale of the defending troops collapsed, and the Lady Otsuya (Kageto's widow and an aunt of Oda Nobunaga) entered into negotiations with Nobutomo.

With the fulfillment of treaty stipulations, Nobutomo made Iwamura Castle his headquarters and a front-line defensive position from which he could support the Takeda.

[5] In 1575, Katsuyori lost the Battle of Nagashino, a disaster for the Takeda clan[6] which left Nobutomo at Iwamura Castle without support.

Under repeated siege by Oda Nobutada, the forces under Nobutomo managed to hold the castle until November when Nobunaga swept in with the main army.