Save your time and go read the manga instead
The male population is extinct in Japan. Thus, even queens end up being mistresses, forced to give birth to male children so the population of the country comes under control. Men, on the other hand, get sold as slaves at throwaway prices.
Ooku, inspired from manga, has a pretty bright start with Sadakichi running off to the jungle. Set in Kanto, the story talks about a world ruled by women and everything doesn't seem as bad.
The story follows the deep secrets of Ooku. It is neither new nor interesting.
The series has a feel-good factor as it progresses. It is a fairytale story combined with scenes which depict cruel realities too. From simply keeping the labour so they don't protest to love not being enough to conceive a child, many topics are openly addressed in this anime.
The mystery rises with the music in the last episodes. The tale that the makers try telling gets clearer each time.
However, there is no mystery. It is simply the transformation in personalities owing to the massive responsibilities.
You can easily skip this show. The manga was released back when the concept was never and in the current times, an anime on it does not have the kind of impact as expected. Thus, read the manga instead if you really want to know the done-and-dusted story.