The Riddle of Zorfendorf Castle

Eric and his friends learn that Zorfendorf Castle holds a secret which they must prevent from falling into the hands of Emperor Ko. There's only one passage between the Upper World and Droon, the magical rainbow staircase. At least, that's what Eric and his friends have always believed. But now Ko and his beasts are attacking Zorfendorf Castle, a magical fortress that's hiding a huge secret: the Fifth River, another passage to the Upper World. Ko will do anything to get to the river, and to the Upper World. And Eric, Julie, Neal, and Keeah will do anything to stop him!