Galeazzo Ciano
He was a close friend of Benito Mussolini's son-in-law Galeazzo Ciano, who was the Italian Minister of Foreign Affairs. Ciano was involved in ousting Mussolini in July 1943 after the Allied invasion of Sicily. Castellano sided with Ciano and had a prominent role in the events that led up to the fall of the fascist regime. He organised the arrest of Mussolini.