Before Takeda Shingen arrived, the site was a simple fortification built by a local warlord named Oi Mitsutada during the Muromachi period. Shingen's redesign through Kansuke transformed it into a strategic stronghold controlling a vital mountain corridor. But Shingen's Takeda clan fell to Oda Nobunaga in 1582, and Komoro passed to Takigawa Kazumasu, one of Nobunaga's generals. When Nobunaga was assassinated in the Honno-ji incident that same year, the castle briefly fell to the Later Hojo clan of Odawara. After 1590 and the Battle of Odawara, Tokugawa Ieyasu claimed the region and assigned Komoro to Sengoku Hidehisa as a 50,000 koku domain. Hidehisa reinforced the walls and reorganized the layout around a central bailey flanked by north and south baileys. His son, Sengoku Tadamasa, improved the castle further, adding the foundation for a three-story tenshukaku -- the main tower that would become the castle's crown.

In 1626, lightning struck the donjon and it burned to the ground. It was never rebuilt. That absence defines Komoro today -- a castle whose most prominent feature is the stone foundation of a tower that hasn't stood for four centuries. The domain passed through a procession of ruling clans: the Hisamatsu branch of the Matsudaira from 1624 to 1648, the Aoyama from 1648 to 1662, the Sakai from 1662 to 1679, the Nishio from 1679 to 1682, and the Ishikawa from 1682 to 1702. In 1702, a junior branch of the Makino clan took control and held Komoro until the end of the Edo period. In 1742, severe flooding damaged the castle. The San-no-mon gate, one of the castle's key defensive structures, had to be rebuilt in 1765 after that flood. Following the Meiji Restoration, Komoro Castle was abandoned. In 1871, with the abolition of the feudal han system, most remaining structures were demolished or donated to nearby Buddhist temples.

What survived is now Kaikoen -- the "nostalgic park" -- and the name captures the spirit of the place. Two original gates still stand: the Otemon, dating from the early Edo period, and the rebuilt San-no-mon, both designated Important Cultural Properties of Japan. Two additional gates survive at Buddhist temples within the city, and a portion of the central palace structure is in private hands in the nearby city of Tomi. The Shinano Railway Line cuts directly through the former castle grounds, isolating the Otemon from the other structures -- a modern railway bisecting feudal ruins. Within the park, the castle grounds host a small zoo that has operated since 1926, an amusement park, and a museum dedicated to the novelist Shimazaki Toson, who spent a formative period teaching in Komoro and set portions of his work in the area. In 2006, the Japan Castle Foundation recognized Komoro as one of the 100 Fine Castles of Japan.

Kaikoen is designated as one of Japan's Top 100 Cherry Blossom Viewing Sites, and in spring the park transforms. Some of the cherry trees are said to be centuries old, their gnarled branches spreading over stone walls and defensive embankments. The Komoro-yae-benishidare, a weeping cherry variety found only in Komoro, blooms in a deep purplish red that gradually fades as the petals open. The blossoms arrive later here than in the lowlands -- the castle sits at an elevation where spring comes slowly -- and visitors who have already seen the season pass through Tokyo and Kyoto make a second pilgrimage to catch the late display. In autumn, the maples take over, and the park's canopy turns red and gold against the old stone foundations. From the bluffs, the Chikuma River is visible below, and on clear days the mountains of Nagano rise beyond. The hole in the ground where Kansuke built his fortress has become a garden.