The Enchanted Castle was written by Edith Nesbit. It was first published in 1907, so you wont find it in the 'new books' section of your library :D. The novel was written in such a way that whenever it started winding down something new and exciting happened. It seemed to me that Nesbit did not want her book to end. At times it dragged a bit, but do not let that keep you from reading The Enchanted Castle; the story itself is pretty good.
Gerald, Jimmy and Kathleen are siblings living in England at about the time the book was written. Their cousin gets sick just in time for the holidays, so the the children cannot go home lest they get sick too. The French Governess at Kathleen's school had been planning on staying at the school anyway, so she agrees when asked to watch the three children. The children convince her to let them take their lunch out of doors and eat in the woods, unchaperoned. While on the path they get lost and soon stumble upon a wonderful garden and a castle. Following the clue they find a princess asleep on the bench in the middle of the maze. After they wake her, she feeds them a most interesting meal and brings them to a secret toom. In this room she points out a magical ring she clams will turn the wearer invisible. And then the adventures begin.