Incarceron by Catherine Fisher is about a missing prince and his betrothed, Claudia, trying to find him. She happens to be the Warden's daughter. The Warden runs the legendary prison — a well-hidden world within a world.
As there are two worlds: that of the prison (Incarceron) and that of the outside (Realm), there are parallel stories. In Realm, there is Claudia trying to do anything to avoid marrying a man she doesn't love while trying to rescue the man she does. Inside there is Finn who has gotten tired of just surviving and wants to follow the legend of Sapphique and escape the prison.
The mystery though of what Incareron is and where it is, gets in the way of Claudia and Finn's stories. The most frustrating aspect of this mystery is that it becomes obvious to the reader long before either of the two put the pieces together. By keeping the mystery a mystery to them, Fisher is prevented from expanding her world building.
That said, it's still a page turner. Although it's over 400 pages, those pages go quickly. It can easily be read in a weekend.