Tuesdays at the castle - George, Jessica Day 1976- (Bloomsbury, 2011)
Sequel: Wednesdays in the tower. -- Eleven-year-old Princess Celie lives with her parents, the king and queen, and her brothers and sister at Castle Glower, which adds rooms or stairways or secret passageways most every Tuesday, and when the king and queen are ambushed while travelling, it is up to Celie--the castle's favorite--with her secret knowledge of its never-ending twists and turns, to protect their home and save their kingdom.