He is, in his early days, in the service of Ōtomo Sōrin, a daimyō chief of a clan on the island of Kyūshū.
Chikatsugu was a samurai converted to Christianity by the name of Don Paulo.
Such an environment drew many Christians to this area and also brought Western culture to this place.
Chikatsugu's brother Shiga Chikanori, who was the former leader of the Shiga clan, moved to Shimazu's side and participated as a guide when Shimazu Yoshihiro marched towards Higo Prefecture's Oka Castle in October 1585.
He distinguished himself in 1586, securing the defense of his castle, resisting invasion attempts by the Shimazu clan until the arrival of Toyotomi Hideyoshi's forces during his Kyushu campaign.