"An enourmous medieval-looking castle sat on the cloudy horizon. It had huge stone bricks, several flat towers, and a gigantic wooden door."
The Giant's Castle is at the top of Jack's Beanstalk in the Red Riding Hood kingdom. Jack climbed up the beanstalk as a young man to look for the woman he loved, Goldilocks. He took a golden harp from the castle and killed the giant by chopping down the beanstalk, causing the giant to fall to his death.
When the twins need to go up to the Giant's Castle in The Enchantress Returns, they go to the Traveling Tradesman to ask for new magic beans to replant the beanstalk. It works, and the castle is still to be found in the clouds. Though the giant is dead, his giant cat still lives there, feeding on birds, and it attacks our heroes.
Inside, the castle is decorated with harp-inspired artwork - and so Alex and Conner realize that the giant's most prized posession (which they need to build the Wand of Wonderment) is the golden harp Jack took.
"The portrait was just one of several different artworks inspired by the magic harp. There were statues and sculptures of her displayed around the room; oil and finger paintings of her covered the walls. Even the backs of the chairs had the harp's silhouette carved into them."
In Beyond the Kingdoms, Mother Goose goes up to live in the castle for a while, accompanied by Lester. She cleans up the bird carcasses and makes the castle look 'lived in' again. She names the giant cat "George Clooney". She leaves the castle and returns to the Land of Stories when Alex needs her help.