Ōiso Jōyama Park (大磯城山公園)

Oiso Joyama Park's South Gate

The saddle-like terrain near the South Gate

Oiso Joyama Park (大磯城山公園) is a Kanagawa prefectural city park in Oiso, Naka District, built on the former site of a villa owned by the head of the Mitsui conglomerate.

The name Joyama (城山), meaning "castle mountain," comes from Koiso Castle (小磯城), where Echigo Goroshiro, a retainer of Nagao Kageharu (1443–1514), is said to have barricaded himself during the Nagao Kageharu Rebellion (1476–1480) before the castle was attacked and captured by Ota Dokan (1432–1486).

The exact location of Koiso Castle remains uncertain, but some suggest that the saddle-like terrain near the South Gate (南門) may have been part of the castle's horikiri.

Horizontal tombs

Archaeological discoveries have also been made within the park, including several relics from the Jomon period (14,000–1000 BC) and numerous horizontal tombs containing pottery and other artifacts. Some of these items are displayed at the Oiso Municipal Museum (大磯町郷土資料館) in the park.

Jozan-an

In 1898, the head of the Mitsui family built a villa here and developed the surrounding garden. One of the structures on the site was Jozan-an (城山庵), a building constructed using ancient materials gathered from famous temples and shrines across Japan. Beside Jozan-an lies a Japanese garden called Momiji Plaza (もみじの広場), which surrounds Fudo Pond (不動池). Every year around late November, the garden hosts a nighttime light-up event.

Koyurugi no Taki flowing into Fodo Pond

Over time, many of the Mitsui facilities were relocated, and the area eventually fell into disrepair. However, after strong public interest in preserving the site, it was reopened as Oiso Joyama Park in 1987.

Momiji Plaza and Fudo Pond

From the observation deck, visitors can enjoy the view of Mount Fuji. Next to the deck is a small stone boat monument with a tiny shrine behind it.

The Kitakura (North Storehouse), now used as a gallery, was built in 1941 and remains one of the few surviving historical structures on the grounds, along with the Higashikura (East Storehouse).

Kitamura Gallery

Higashikura

In September 22, 2013, the Former Residence of Yoshida Shigeru (旧吉田茂邸) was added to Oiso Joyama Park.