The Enchanted Castle - E. Nesbit

Three children, forced to remain at school during the holidays, go in search of adventure. What they find is a magic castle straight out of a fairy tale, complete with an enchanted princess at the center of a maze. Or is it? The castle turns out to be just a country estate, and the princess is only the housekeeper's niece, playing at dressing up. But the magic ring she shows them proves -- to her surprise and horror -- to really be magic. Soon they are caught in an adventure where statues come alive, lost lovers are reunited, and wishes can be granted -- but always for a price. (Summary by Peter Eastman)