After the Siege of Odawara in 1590, Toyotomi Hideyoshi's Oshu Shioki assigned Gamo Ujisato a 420,000-koku fief in Aizu (later expanded to 920,000 koku), and he took over Kurokawa Castle from Date Masamune. Ujisato immediately launched a major renovation to transform it into a modern castle, constructing a new donjon and renaming it 'Wakamatsu Castle' (later Tsurugajo). The grid layout of the castle town and the very place-name 'Wakamatsu' are credited to Ujisato's initiative. As a Christian daimyo, he also invited missionaries and oversaw a flourishing of Christian culture in Aizu. However, he died of illness in 1595 at just 40 years old. The castle town and the modern face of Aizu that Ujisato established are carried forward in the Tsurugajo of today.

After Gamo Ujisato, Uesugi Kagekatsu took up residence at Tsurugajo, and following the Battle of Sekigahara, Hoshina Masayuki made it his castle.

1589年の摺上原の戦いで蘆名氏を滅ぼした政宗は鶴ヶ城（当時は黒川城）を制圧し会津を領有した。しかし翌年の奥州仕置で秀吉に会津を没収され、政宗の夢は頓挫した。