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 and Gandhi in the play Gandhi at the Tricycle theatre London.

His screen debut was in Michelangelo Antonioni’s 1966 film Blowup playing David Hemmings’ artist friend, Bill. In 1968 as the plotting Prince Geoffrey in the big-screen adaptation of The Lion in Winter, he garnered much praise and it set him on his way as a quality supporting player in London and Hollywood. According to Rotten Tomatoes, The Lion in Winter is Castle’s “highest-rated” film. Also in 1967, he appeared in the British TV Series, The Prisoner as Number 12, a sympathetic guardian in the episode, entitled “The General”.

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.