Fukushima Castle, Mountain fortress in Kiso Valley, Japan

Fukushima Castle is a mountain fortress in the Kiso Valley built across three linked defensive terraces on Mount Shiroyama. The site features earthen ramparts, trenches, and fortified lines that follow the steep slopes to create layered protection.

The fortress was founded in the 1530s by Kiso Yoshiyasu as a regional stronghold controlling this mountain passage. It fell to Takeda Shingen in 1554 after a siege that broke the defenders' resistance through isolation and supply pressure.

The castle is also called Kiso-Fukushima Castle to distinguish it from other castles of the same name. Kiso Yoshiyasu built it in the Sengoku period, and Kiso Yoshimasa used it as his residence, keeping Ue-no-dan Castle as his daily residence and the mountain castle as his last redoubt. In 1582, after the Honnoji Incident, Yoshimasa tried to kill Mori Nagayoshi during his retreat, but Nagayoshi stormed the castle and took Yoshimasa's son Iwamatsumaru hostage.

The ruins are reached by hiking trails that begin from several starting points in the surrounding area and lead upward to the terraces. Wear sturdy footwear since the paths are steep and can be slippery when wet or after rain.

In 1582, after the Honnōji Incident, Kiso Yoshimasa tried to ambush Mori Nagayoshi as he retreated to Mino, but Nagayoshi discovered the plan, entered Fukushima Castle, and took Yoshimasa's son Iwasumaru hostage. Yoshimasa then had to ask other lords not to attack the Mori and instead help Nagayoshi withdraw.