Katō Kiyomasa was a military man first and last, outlawing even the recitation of poetry, putting the martial arts above all else. His precepts show the single-mindedness and Spartan attitudes of the man, they demonstrate emphatically that the warrior's first duty in the early 17th century was simply to 'grasp the sword and die'. Contemporary accounts of Katō describe him as awe-inspiring, yet not unfriendly, and a natural leader of men.
Kiyomasa was a renowned castle-builder. During the Imjin war he built several strategic Japanese-style castles in the territories he conquered. Ulsan castle was one of such fortresses and the site of Kiyomasa's most famous battle — the Siege of Ulsan on December 22, 1597. Kiyomasa led the defense of the castle, successfully holding at bay Chinese general Yang Hao's army, which numbered 60,000. He defended the castle until November 23, 1598. However, his bravery was not reported to Hideyoshi by his rival and superior Ishida Mitsunari. Hideyoshi recalled him to Kyōto. After Hideyoshi's death, Kiyomasa clashed with Mitsunari, and started approaching Tokugawa Ieyasu.
During the Battle of Sekigahara, Kiyomasa remained in Kyūshū. Believing that the Toyotomi would fall without the help of the Tokugawa (and the hatred he had of Ishida Mitsunari commanding the Toyotomi forces), he sided with the eastern army of Tokugawa Ieyasu. For his loyalty to the Tokugawa, Kiyomasa was rewarded with the former territories of his rival Konishi (who had sided with Ishida), which when added to his existing territory, increased the Kumamoto domain to around 530,000 koku.