The Enchanted Castle — Edith Nesbit / E. Nesbit

There is a curtain, thin as gossamer, clear as glass, strong as iron, that hangs forever between the world of magic and the world that seems to us to be real. And when once people have found one of the little weak spots in that curtain which are marked by magic rings and amulets, and the like, almost anything may happen.

When Gerald, Jimmy, and Kathleen set out in search of adventure, they stumble upon a mysterious castle with a beautiful princess asleep in the garden. The princess is really Mabel, the housekeeper's niece, who is only pretending to be royalty. But when she shows them a secret room filled with treasure where they discover a magical ring, enchantment becomes a reality. The children soon realize they'll need all of the bravery and ingenuity they can muster to contend with the magic enveloping the enchanted castle.