Castle's first appearance was as Westmoreland on stage in Henry V on 5 June 1964, at the Open Air Theatre, Regent's Park. His first Broadway theatre appearance was in February 1970, as Jos in the short-lived musical Georgy.
Among Castle's stage performances was his role as Oswald in the Royal Shakespeare Company's revival of Ibsen's "Ghosts" in 1967, with Dame Peggy Ashcroft as Oswald's mother Mrs Alving.
Castle played the role of Octavius Caesar in Charlton Heston's poorly reviewed version of Antony and Cleopatra (1972), a film that received no US release at all.
Castle is married to the Writer Maggie Wadey. In a biographical essay for Who's Who in the Theatre in 1981 he listed his favourite parts as Luther, Hamlet and O'Riordan.
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). He played Inspector Craddock in an adaptation of the Agatha Christie story "A Murder is Announced" (1985), a role he recreated in the Miss Marple mystery "The Mirror Crack'd from Side to Side" (1992). He also played the title role in the 2000 made-for-TV version of Christie's Lord Edgware Dies. In 1990 Castle starred as Superintendent George Thorne in the BBC's radio adaptations of John Penn's novels. Castle appeared in other TV series, including I Claudius, Ben Hall, and Lost Empires.
