Château d'Asahi Yamashiro, Mountain fortress in Sakata, Japan

Château d'Asahi Yamashiro is a medieval mountain castle in the hills east of Sakata, Yamagata Prefecture, on a summit above the Yadaregawa River. The flat main enclosure looks over the northern Shonai Plain; the eastern end has two dry moats and a square gate, and damaged stone-wall traces remain near the second enclosure.

The Ikeda clan built this military stronghold around 1335 during the Northern and Southern Courts period. It served as a key defensive position in the regional conflicts that shaped the area's politics during the 14th century.

The name Yadaregawa, for the river below the castle, comes from a legend of the Northern and Southern Courts period: warriors shot arrows at each other across the water, and afterwards arrows drifted downstream, giving the river its name. The upper Yadaregawa in the Oishi district keeps place names and monuments linked to warriors, such as the Samurai Waterfall and Hachiman Shrine.

The site is accessible by walking trails that wind through the different bailey sections toward the former central compound. Sturdy footwear is recommended because the terrain is uneven and the paths can be steep in places.

The river below the castle is called Yadaregawa, not Yamagawa. According to local tradition, warriors once exchanged arrows across it during a battle in the Northern and Southern Courts period; afterwards the arrows drifted downstream, giving the river its name, which survives today as both a river name and a place name.