The phrase "King of the Castle" is an idiomatic expression that means being in a position of power, control or dominance. It can refer to someone who has authority over others and exercises it with confidence and assertiveness. The term originated from the game of children's playground tag where one player would be designated as "king" by touching another player, thereby making them subject to their rule until they were "captured." In this sense, being "King of the Castle" means having the upper hand and being in control.