(From the publisher):
Eric and his friends have long known about the mysterious and beautiful Zorfendorf Castle. But when Emperor Ko launches a surprise attack, the kids discover that the castle is not all it seems. Hidden deep under Zorfendorf is one of the most baffling secrets in all of Droon. Ko will do anything to discover that secret. And Eric, Julie, Neal, and Keeah will do anything to stop him!
Original title: The Riddle of Zorfendorf Castle
Genre: Fiction→ Children & Young Adult→ Fantasy→ Fantasy Worlds