What is the setting of Sapphique by Catherine Fisher?

The novel has two primary settings. One setting is in the world of Incarceron. The other is in the world of the realm. The setting is a combination of what we think of as reality and fantasy. The main characters are trapped in the worlds in which they live and are unable to get to their loved ones that are trapped in the separate world. They are all hunting for the portal, which is the door that allows them to move between these two worlds.