The Riddle of Zorfendorf Castle: The Riddle of Zorfendorf Castle

Both my kids liked these as much as the Magic Tree House series.

The kids, Eric, Neal, and Julie have always known about Zorfendorf Castle. However, it is only when Emperor Ko, the stepfather of Lord Sparr, launches a surprise attack on the city that the kids know that the mysterious city has its own secrets. Before the kids only knew that there was only one passage between the Upper World and Droon,the magical rainbow staircase that they enter Droon through each time. But now everything has changed. Under Zorfendorf Castle lies a huge secrets, one that Ko is desperate to use, the Fifth River, another way into the Upper World. The kids know they must stop Ko from getting to the river, but how? Once again they must come up with a plan to save the day. Another wonderful adventure. Can't wait for further installments.