There are four books in Wind Up Knight; the farther you go, the trickier it gets. Each book has 12 levels, each with a hidden card that is reasonably tricky to achieve. There is one, special level every book called a Knightmare. They are almost always easily the hardest level in the book. However, you can only play these Knightmares if you complete every level in the book with an A rank or better. The best rank is S, which you get if you get all the coins and the hidden card.