Edith Nesbit 
The Enchanted Castle [EPUB ebook] 

Gerald, James and Kathleen, discover an enchanted castle while exploring during the school holidays. The lake, groves and marble statues, make a fairy-tale setting. In the middle of the maze in the garden they find a sleeping fairy-tale princess. The ‘princess’ tells them that the castle is full of magic, and they believe her. She shows them the treasures of the castle, including a ring she says is a ring of invisibility, The children soon discover that the ring has other magical powers.