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

With nearly 2 million books in print, this Little Apple series is H-O-T, hot. The SECRET is out -- DROON is the series that kids, parents, and teachers are talking about! 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!