Born Ulv Quarzell in Bogotá, Castel moved to Europe as a young man. Interested in acting from an early age, he attended the Centro Sperimentale di Cinematografia, but was quickly kicked out. His first movie role was an uncredited extra in The Leopard (1963). But two years later, he would catch his big break when he starred in Fists in the Pocket. His performance as the epileptic Alessandro, who murders his mother and his brother, was acclaimed all over the world. His next role to gain international prominence was probably as Jeff, the temperamental bisexual film director in Beware of a Holy Whore (1971), directed by Rainer Werner Fassbinder. Fassbinder himself portrayed the film's producer.
In the following decades, Castel worked with some of the other important European directors of the era, including Pier Paolo Pasolini, Wim Wenders, Philippe Garrel, and Olivier Assayas. Though the quality of the films he acted in were quite disparate, ranging from arthouse films to cheap exploitation, Castel always chose roles that reflected his militant leftist beliefs. He has a son from the actress Marcella Michelangeli. Castel remains active in his profession.