The Xanadu Adventure — Lloyd Alexander

Vesper and her friends are sailing off to the newly discovered archeological site of Troy. But danger is close behind -- and the party is tricked and imprisoned in a fabulous palace called Xanadu. Xanadu's master is none other than Vesper's archenemy, Dr. Helvitius, who has a diabolical scheme to dominate the world. His first goal is to destroy Vesper, who has thwarted him once too often. Escape seems impossible. But if anyone can do it, Vesper can!