king of the castle

noun mainly British

a children's game in which each child attempts to stand alone on a mound, sandcastle, etc, by pushing other children off it
informal a person who is in a commanding or superior position

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012