Court of Crimson by Helen Scott

Abandoned. Unwanted. Orphan.
In a time of war between mankind and fae, no one ventures outside of the safety of the human kingdom. Abandoned in the furthest corner of Amnestia, in a convent long forgotten by most, Cress grows up thinking she’s nothing more than a human of horrendous skill and grace.
One moment, the quiet countryside of Amnestia where she’s been raised her whole life is devoid of any danger and the next, it’s there. Powered by a strange magical spell, a castle appears seemingly from nowhere. Ruled by a court of fae few humans had ever laid eyes on and lived to tell the tale, Cress stumbles right into the center of three royal princes. There, she discovers that perhaps she’s not a human at all, but a changeling–a fae child left behind in the ancient tradition of an extinct court.
In the face of possible execution, she better hope she’s a fae, because the worst thing a girl can be in the court of crimson is human.