Kasugayama Castle, Mountain fortress in Joetsu, Japan.

Kasugayama Castle is a medieval mountain castle on the summit of Mount Kasugayama in Joetsu, Niigata Prefecture. The site occupies a wooded hill whose slopes are covered in dense green trees, with a grassy clearing at the top where a few small structures and stones stand. Illustrations often show stone walls and a keep, but no stone walls or roof tiles have been excavated; the defences were earthen banks, moats and forts on the surrounding ridges.

The castle began in the Nanboku-chō period as a stronghold of the Uesugi shugo, and in 1507 Nagao Tamekage took it after expelling Uesugi Fusanori. It served four generations of the Nagao and Uesugi, including Uesugi Kenshin, until Uesugi Kagekatsu left for Aizu in 1598. The Hori clan moved to Fukushima Castle in 1607, ending Kasugayama's role.

The fortress takes its name from the nearby Kasuga shrine and sits embedded in the wooded slopes rising above the city. Visitors today walk trails once used by samurai troops, with cedars and beeches lining the old defense lines.

The climb to the ruins takes around 40 minutes and involves uneven ground that can become slippery in wet weather. Sturdy footwear and drinking water are advisable, as there are no facilities on the mountain.

The layout avoids tall stone towers and instead relies on low platforms that follow the curves of the terrain. This design made the fortress less visible from below and harder for attackers to navigate in the dense forest.