Most of the best of the "Choose Your Own Adventure" series (in which readers are asked to make periodic choices that help determine the outcome of the story)were written by its originator, Edward Packard, and "Forbidden Castle" is definitely one of Packard's better efforts. (I think his best may have "Who Killed Harlowe Thrombey?") It begins with the protagonist - you - returning to the Cave of Time (book #1 in the series) and ending up in medieval England, where the ruthless King has offered a fortune to anyone who can solve the riddle that will lead to a Forbidden Castle of great power. The riddle is a clever one with a logical explanation, and the quest for the castle proves a dangerous and ultimately rewarding one, though not in the way one might expect. Young readers (say, 3rd-5th grade) will want to read the book many times to discover all of the endings, and enjoy the historical fiction in which sensible decisions really do take you somewhere. The book features appealing illustrations by Paul Granger.