- "Your barons are no better than the King if they take aid from the French."
- ―William of Cassingham justifying his allegiance to John.[src]
William of Cassingham (died 1257), later known as Willikin of the Weald, was a Master Assassin of the English Brotherhood of Assassins and an expert archer who fought for King John during the First Barons' War, as opposed to the barons supported by Robert Fitzwalter and the other members of the English Brotherhood.
Rallying a guerrilla force against the English barons and their ally, Prince Louis VIII of France, William favored John's authority to that of the baronial rule, despite objecting to his despotic approach; as a supporter of John's son, Henry III, William saw this avenue as the lesser of two evils.