Ribbon no Kishi

A medieval fantasy retelling centers on a princess who is raised as a boy to secure her claim to the throne, as female heirs are not permitted to rule. From birth, she is presented as a male heir by her father, the king, to avoid the succession passing to Duke Duralumon, a tyrannical noble. The king believes the duke would oppress the kingdom if crowned, and so resorts to deception to keep power from falling into his hands.