The Game by Diana Wynne Jones
A dark castle in Ireland, a mysterious, secret game, and a host of magnificent characters Diana Wynne Jones latest magical offering is a splendid blend of the familiar and the unusual.
Hayley's parents disappeared when she was a baby, so she has been brought up by her grandparents. Then one day she is packed off to Ireland to live with her aunts and a whole host of cousins she never new about! Here she is introduced to "the game" which involves adventures in the forbidden "mythosphere". And here also is where Hayley discovers the truth about her family.