I read this on the recommendation of the same friend as The Winter Prince, but this one wasn't as much of a success. I really liked the first half, with its mysterious urgency and perils, as we struggle, like Callie and West, to understand the rules of this new world and the reason for their journey. Everything through the mysterious castle was marvelous and all of a piece, but once we got to Callie and West's journey through the desert, the tension dropped, and the allegorical ending was a bit obvious for me. The gender roles sometimes rubbed me the wrong way, but the book and the characters seemed aware of the that.
The ending reminded me of a less brutal The Amber Spyglass (a book I love to bits) and I wonder if Seaward influenced that ending.