The Riddle of Zorfendorf Castle (The Secrets of Droon #25‪)‬

A hidden door. A magical staircase. Discover the world of Droon!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!