MAOYU is an anime series based on a light novel series by Mamare Touno; the story was originally posted in a play format on the 2channel messageboard in 2009.
MAOYU is set 15 years before the war between the humans and the demons began. At the beginning of the first episode, the humans' greatest hero suddenly leaves his companions and charges toward the Demon King's castle. The hero is intent on vanquishing the Demon King and bringing the war to an end.
When the hero arrives at the castle, he is surprised to find an attractive woman in the throne room; she claims to be the Demon King. The hero doesn't believe her at first. After he's convinced of her identity, he originally intends to follow through with his plan. But after the Demon King explains how much chaos will be brought upon the world if the war suddenly ends, the hero ends up forming an alliance with the Demon King. Together, they travel the Human Realm together, with the Demon King disguising herself as a human noblewoman known as the Crimson Scholar.
By the time I finished watching the first episode of MAOYU, I thought there was potential for the premise that was presented. Unfortunately, the first episode is written in such a way that it's primarily an "info dump," and not much actually happens during the episode. While I understand that the first episode is supposed to establish the characters, world, and situation of the series, other series have done this successfully without having to spend most of the first episode's runtime dumping a ton of information on the viewer with little to no action taking place.
When it comes to the animation, I have to say that there are some very well-done backgrounds. Unfortunately, the animation utilized for the characters isn't as impressive for the most part. It's not that the animation for the characters is necessarily bad, but the backgrounds tend to stand out to the viewer more than the characters do.
Content-wise, there is some "fanservice" in regards to the Demon King. She is a woman with larger breasts who wears a rather low-cut dress, and several times in the first episode, there is an emphasis placed on her bouncing breasts. Overall, though, this "fanservice" wasn't nearly "in your face" as the "fanservice" in Medaka Box, and it didn't distract from the story quite as much as it could have.
With the "fanservice" and concept of this series, I would personally recommend MAOYU to anime viewers who are 15 or 16 years of age and older.