Yogaisan Castle, Mountain fortress in Kofu, Japan

Yōgaiyama Castle is a Sengoku-period yamajiro on a narrow ridge of Mount Yōgai, rising to 780 meters above Kōfu. About 20 terraces are cut into the slope, connected by a zigzagging path that exposes attackers to defenders above. The inner bailey, a rectangular 80 by 30 meters, sits at the summit and was enclosed by clay walls with gates on its east and west sides; the castle extends over 500 meters in total.

Built in 1520 by Takeda Nobutora, the fortress served as a strategic retreat during Japan's turbulent Sengoku period of constant warfare. The site marks the early stronghold of the Takeda clan before their rise to regional prominence.

The castle is also called Sekisuiyama Castle, after Sekisui-ji, the temple in the hamlet of Kamisekisui at its foot. Takeda Nobutora's wife, daughter of Ōi Nobusato, took refuge here in 1521 and gave birth to the future Takeda Shingen; he was named Katsuchiyo to mark his father's victory over the Imagawa force. The site is a nationally designated historic site, and signboards mark the main earthworks along the trail.

The site is accessed by hiking up a wooded trail from the base of the hill, requiring sturdy footwear and moderate physical effort. Plan your visit for clear weather, as muddy conditions can make the ascent slippery and visibility limited on misty days.

The castle's name, Yōgaiyama, means "fortress mountain"; it was built as the fallback redoubt of Tsutsujigasaki Yakata, Takeda Nobutora's moated residence in Kōfu. Nobutora's wife, a daughter of Ōi Nobusato, took refuge here in 1521 and gave birth to the future Takeda Shingen the day before her husband defeated the Imagawa force.