In 1572, the Takeda organized another campaign against Mikawa province which would culminate in the Battle of Mikatagahara in January 1573. As Takeda Shingen drove south and west, Nobutomo would descend from the north, cutting off an escape route and blocking reinforcements. To accomplish this, Nobutomo laid siege to Iwamura Castle. 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. They agreed on a treaty, and under its terms the castle was surrendered without bloodshed, and Lady Otsuya agreed to marry Nobutomo, thus securing his protection and the safety of the defending troops. Also among the spoils was , the biological son of Oda Nobunaga, adopted son of Kageto, who was then seven years old. Nobutomo sent him to Kai Province as a hostage; the boy would later become known as Oda Katsunaga. 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.