Scrapped Princess (2003) IMDb
A prophecy in an otherwise fairly idyllic Renaissance world states that a certain person will destroy everything unless she is killed before she turns sixteen. Her older brother and sister—a warrior and mage, respectively—have decided not to let anybody do that. They try to protect her on a long journey.
Fantasy which turns out to have heavy SF elements. Humanity’s imprisonment is quite neat, and the nature of the cage does explain in part why the world is so nice and campy, but in the end, there really is no good reason why the prophecy exists. Mood twists guaranteed: they’re fairly successful overall. Even characters who are normally stupendously mild-mannered or genki can actually freak out the way they ought to. Some good humour.