It's a thematically rich exploration of female sexuality and the power of the church in 15th century Europe. The heroine's a witch (with magical powers) trying to stop the fighting in the Hundred Years' War. This gets pretty dark in the second half, but it's not trying to crush your spirit or anything. It's funny and entertaining, it has a happy ending and it's downright sunny if you consider the subject matter and the fate a woman could expect in that historical era.
Our heroine is Maria. She's a passionate, kind-hearted girl who'd throw herself under a bus if she thought it was the right thing to do, but everything she does comes from the heart, not the head. She hates fighting. She hates to see suffering. She'll thus send giant magical monsters to stomp through any battlefield she sees, even if she knows that the battle she's preventing would have helped to end the war.
She's not soft or squeamish, though. She's turned her owl familiar into a succubus (Artemis) who undermines the war effort with ten or twenty sex acts a night. On learning that not everyone likes girls, Maria just makes an incubus too. Problem solved, or at least it would have been if she'd been able to make Priapus any genitals. It's not her fault she doesn't know what men's undercarriage looks like.
Nonetheless Maria thinks things are going well... until God intervenes. No, not the church, although they hate her too. (She's a heretic. Witches are for burning, although arranging this is surprisingly bureaucratic.) The Archangel Michael descends from heaven to try to put Maria in her proper place. She's disrupting the natural order by saving lives. War is natural. Violent death is right and proper. It's God's will that mercenaries should go pillaging villages and treating rape as a perk of the job. Maria should stop using her magic where mortals can see her, or else she'll be smited by the wrath of heaven! (Hypothetical translation: her magic is a threat to God's monopoly over supernatural fear.)
Alas, Maria isn't listening to any of that. Michael thus gets annoyed and makes his conditions more specific. One: he's sending his servant Ezekiel to watch her. Two: if she ever has sex, she'll lose her magical powers.
In other words, the church (in heaven) is telling a woman that she has to be a virgin, or she'll be punished. However that's better than the church on Earth, who are evil. They're scary because they've defined themselves as the arbiters of good, so by definition their opponents are wicked and any action taken against them is permissible.
All this is juicy. What makes it better still is how carefully and intelligently it's being executed.
The history is spot-on. I've seen it claimed that it's not just the best anime portrayal of its era, but the best fictional one. War is how some of these people earn a living. Prevent battles and you're damaging their livelihoods and pushing them towards the looting of villages. You'll see empty villages, depopulated by the Black Death in the previous century. Similarly there's nothing glamorous about medieval warfare. Armies are an ill-led collection of arsehole knights, sleazy mercenaries and conscripted peasants who don't know what they're doing and just want to go home.
They're even being accurate with odd-looking stuff like holding a sword by the hilt so you can clobber someone with the handle. (It's called "mordhau".) Ep.3 also touches on the trial and execution of a pig.
It's not a dreary, muddy, so-authentic-that-you-don't-care 15th century, though. It's got dragons, trolls and cute dancing owls. The witches are modern, liberated girls and you can see why the church hates them. Everyone's flawed and no one's one-dimensionally evil, no matter how appalling their deeds. Garfa was born to a mercenary life and has never had anything. The monks believe that they're doing right. Conversely it's true that Maria is pushing her own beliefs on others, reacting to the immediate situation without worrying about what anyone else thinks or what the long-term outcome might be.
It's about female sexuality. In a culture where women are liable to be treated as property and/or the spoils of war, Maria is being herself and growing into her own choices. (It's not just the church. Sexuality is a commodity and there are those who are more than willing to trade it or embrace it. Some of her female acquaintances are telling her flat-out to lose her virginity.) Interestingly, the two most significant male figures in Maria's life have both been feminised. The Archangel Michael's voice is by Kikuko Inoue (!) and (s)he shines so brightly that you can hardly see a body at all, while Joseph is a passive, gentle man whose plot role is the traditional hero's noble love, but gender-switched.
However it's also about religion. The church burns witches, but only when their magic isn't convenient. Priests twist the truth into whatever they want you to believe. They tell you it's a sin to have your life saved by a witch, insisting that everything is God's will as they effectively give their blessing to massacres, rape and judicial murder. (What's more, none of this is made up. History is full of all this, except with no powers and no happy ending for women who were really accused of witchcraft.) "With God's divine protection," chant the soldiers on all sides as they march towards their deaths, then react with fury when Maria actually does some protecting. God never helps anyone, of course. God's trying to bully Maria into being as hands-off as Him.
(Maria's universe isn't monotheistic, by the way. All kinds of mythologies are in there, from Valkyries to Ancient Greece. It's just that Christianity happens to be the most powerful at present, because it has the most followers.)
This is a rich, intelligent show with far more depth than anyone expected from the original manga. No one's merely a hate figure, while Maria has a lot of growing to do too. At first, it's highly entertaining. Later it gets less so as the story gets darker, but it's never depressing or oppressive. It's a show to respect and appreciate.