Answer:

william

Answer	Crossword Clue
WILLIAM	Charles's older son
WILLIAM	King in 1066
WILLIAM	Singer whose stage name includes two periods

WILLIAM	Tell of Shatner
WILLIAM	Winner of 1066
WILLIAM	Prince Harry's brother
WILLIAM	Several lines of European kings.
WILLIAM	Tell, for one.
WILLIAM	Orangeman.
WILLIAM	Guglielmo, in America
WILLIAM	Prince who married Kate Middleton

WILLIAM	Leader at the Battle of Hastings
WILLIAM	Taft or Tell
WILLIAM	Shortz's first name, or his existential declaration?
WILLIAM	2000s music star who writes his name with four dots
WILLIAM	British royal
WILLIAM	Royal Charlotte's father
WILLIAM	Member of the British royal family