The Girl in the Tower by Katherine Arden (Winternight Trilogy #2)
The magical adventure begun in The Bear and the Nightingalecontinues as brave Vasya, now a young woman, is forced to choose between marriage or life in a convent and instead flees her home—but soon finds herself called upon to help defend the city of Moscow when it comes under siege.
Orphaned and cast out as a witch by her village, Vasya’s options are few: resign herself to life in a convent, or allow her older sister to make her a match with a Moscovite prince. Both doom her to life in a tower, cut off from the vast world she longs to explore. So instead she chooses adventure, disguising herself as a boy and riding her horse into the woods. When a battle with some bandits who have been terrorizing the countryside earns her the admiration of the Grand Prince of Moscow, she must carefully guard the secret of her gender to remain in his good graces—even as she realizes his kingdom is under threat from mysterious forces only she will be able to stop.