The Seizansō (西山荘) is located in Hitachiōta in Ibaraki Prefecture, about twenty kilometres north of the prefectural capital Mito. When the legendary second daimyō of Mito, Tokugawa Mitsukuni retired from his active duties as vice-shōgun in the winter of 1690, handing over the administration of the domain to his nephew Tsunaeda, the emperor conferred the title of "provisional vice-councillor of state" (権中納言 gon-chūnagon), also called kōmon, on him. Henceforward, Mitsukuni was known under the respectful sobriquet of "Mito Kōmon" (水戸黄門).

In May 1691, Mitsukuni moved into the Seizan-sō, a modest villa with a thatched roof at the foot of a mountain where he spent the rest of his days pursuing historical studies and herbal medicine. The name Seizansō ("Western Hill Palace") was inspired by a poem of the Chinese "recluse" poet Tao Yuanming (365–427 CE). The roof was thatched with miscanthus; irises were planted on top of the roof and acted as natural hygrometers: when humidity was low, and they drooped, the roof was sprinkled with water to prevent fire.

Mitsukuni devoted his time to compiling his great historic treaty, the Dai Nihonshi (大日本史, lit. “Great History of Japan”), and cultivating medicinal plants in his garden. He published a compendium on 397 different types of herbs and distributed it to the general public.

Mito Kōmon died on December 6, 1700, in his bedroom at the Seizansō. In 1817, the house was destroyed by wildfire and rebuilt by the eighth Mito daimyō, Tokugawa Narinobu (1797-1829), two years later, though on a smaller scale. In 1974, the Seizansō was designated a Historical Site of Ibaraki Prefecture. It is now under the supervision of the Tokugawa Foundation.

The Kunugi-mon (the Oak Gate), consisting of two pillars of Quercus acutissima symbolising Mitsukuni's proclivity for a simple life

The entrance to the Seizansō in Hitachiota with the Onari-michi, Mitsukuni's alley

The restored main building of the Seizansō

Detail of the main building at the Seizansō

A life-size figure of Tokugawa Mitsukuni, better known as "Mito Kōmon" (水戸黄門)

Photos of actors on the set of the jidaigeki (時代劇, “period drama” or historical drama) "Mito Kōmon" produced by TBS (Tokyo Broadcasting System) between 1969 and December 2011

The Kunugi-mon at the Seizansō with the two pillars of Quercus acutissima.

Access and admission:

By train: 50 minutes from Mito by JR Suigun Line to JR Hitachiōta Station, then by bus (bound for Jogu Tashiro), a 5-minute walk from Seizansō bus stop.

Admission: 800 JPY (adults), 600 JPY (children); open daily 09:00-16:00 (last entry at 15:30) except Monday (unless it is a public holiday).