Located in what is now Mihara City, Hiroshima Prefecture, it encompassed the southern part of Bingo Province.

For more than two hundred years the daimyō of Mihara were a collateral branch of the Asano clan who held Hiroshima Domain.

Mihara Castle was built 1567 by Kobayakawa Takakage, the third son of the famous warlord Mōri Motonari who controlled most of Aki and Bingo Provinces.

[2] It was a large scale castle with 3 baileys, 32 sumi yagura (corner towers), and 14 mon (gates).

It was built on the coast and connected two islands giving rise to the nickname Ukishirō or "floating castle."