king of the castle someone holding a pre-eminent position, rank, or place; a dominant or successful person.
☞ The allusion is to a children's game in which someone stands on top of a mound and dares the other children to pull him or her off. This is accompanied by the rhyme 'I'm the king of the castle. Get down, you dirty rascal!'
2015Guardian Hollywood's king of the castle, Steven Spielberg, has been in subdued mood for some time.