This book has all the elements of a great thriller -- Scotland! a spooky castle! charmed children! -- and Fox strikes the perfect balance of being spooky enough to thrill without going too far overboard and terrifying this book's target audience. The backdrop of WWII adds a nice layer of complexity to the plot of this book, which is clever and well-thought-out, and clearly written with a lot of respect for the children who will eventually read it. Highly enjoyable.