Sentence:  Wainwright, who is composing the music, is partnered with Daniel MacIvor, who is writing the libretto. “Both Daniel and I are used to being <WSD>king of the castle</WSD>,” Rufus confides, “so we need a third party to give us a sense of perspective.” 
Meaning of the word marked with <WSD> tags:  An individual who assumes a position of greater importance, authority, or prominence than others.