He is best known for his film and television work, most notably playing Bill in Michelangelo Antonioni's Blowup (1966) and Geoffrey in The Lion in Winter (1968).
His screen debut was in Michelangelo Antonioni's 1966 film Blowup playing David Hemmings' artist friend, Bill.
In 1968, he portrayed the plotting Prince Geoffrey in the film adaptation of The Lion in Winter, starring Peter O'Toole and Katharine Hepburn.
Castle appeared as Carruthers, the most honourable of a trio of schemers in an episode of Granada Television's series Sherlock Holmes ("The Solitary Cyclist", 1984).
His association with Sherlock Holmes continued with his role as Nigel St Clair in the film version of The Crucifer of Blood (1991).