Because I love this series so much, I find it difficult to write a review for it. Ōoku is an alternate history of Japan where 3/4 of the male population were wiped out due to a pox-like plague. Despite the shortage of men, hundreds of men live in the inner chambers of the palace, to please the shogun. Traditionally male and female roles of the 1700s in Japan are now flipped in society, leaving this place with female shoguns, tradesmen, and heads of family. 
There's a lot of interesting experimentation in this story with masculine and feminine roles, with the remnants of society before the loss of population clearly playing a role. It's difficult to describe without spoiling too much but I find this version of society so interesting and believable that I'm always invested every time I read this series.