Lady Kasuga in the context of "Azuchi–Momoyama period"

Tokugawa Iemitsu (徳川 家光; 12 August 1604 – 8 June 1651) was the third shōgun of the Tokugawa dynasty. He was the eldest son of Tokugawa Hidetada with Oeyo, and the grandson of Tokugawa Ieyasu. Lady Kasuga was his wet nurse, who acted as his political adviser and was at the forefront of shogunate negotiations with the Imperial court. Iemitsu ruled from 1623 to 1651; during this period he crucified Christians, expelled all Europeans from Japan and closed the borders of the country, a foreign policy that continued for over 200 years after its institution.

The Ōoku (大奥, "great interior") was historically the women's quarters of Edo Castle, the section where the women connected to the reigning shōgun resided. Similar areas in the castles of powerful daimyō, such as the Satsuma Domain, were also referred to by this term.

During the reign of the second shogun, Tokugawa Hidetada, Ōoku was established in Edo Castle as a women's room where his official wife (御台所, Midaidokoro), Oeyo, resided. During the reign of the third shogun, Tokugawa Iemitsu, the Ōoku was expanded at the suggestion of his nanny, Lady Kasuga, to ensure the birth of a male heir to the shogun's lineage, and became a vast shogun's harem with nearly 1,000 women working as maidservants.