He is known for his role as Agrippa Postumus in the BBC television adaptation of I, Claudius (1976) and for playing Geoffrey II, Duke of Brittany, in the film The Lion in Winter (1968). He also played Dr Carrasco as well as the prisoner called "The Duke" in the film Man of La Mancha (1972), and the villainous Paul McDagget in RoboCop 3 (1993).
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.
In 1967 he made his screen debut as the plotting Prince Geoffrey in the big-screen adaptation of The Lion in Winter. The role garnered him much praise and set him on his way as a supporting actor 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".
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.
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.
Castle is married to writer Maggie Wadey.
- Blowup (1966) – Bill
- The Lion in Winter (1968) – Geoffrey
- The Promise (1969) – Marat Yestigneyev
- Antony and Cleopatra (1972) – Octavius Caesar
- Made (1972) – Father Dyson
- Man of La Mancha (1972) – Sanson Carrasco / The Duke
- The Incredible Sarah (1976) – Damala
- Eliza Fraser (1976) – Captain Rory McBride
- Eagle's Wing (1979) – The Priest
- Never Never Land (1980) – Jim
- King David (1985) – Abner
- Dealers (1989) – Frank Mallory
- RoboCop 3 (1993) – Paul McDaggett
- Sparrow (1993) – Giuseppe
- Merisairas (1996) – Chief Engineer Josif Mantz
- Gods and Generals (2003) – Old Penn
- I Against I (2012) – Tommy Carmichael
- The Prisoner (1967) – Number Twelve in the episode "The General"
- The Shadow of the Tower (1972) – Thomas Flamank
- Softly, Softly (1973) – Billy Mason
- Warship (1975) – Lieutenant-Commander Peter Tremayne, officer commanding the Royal Navy submarine HMS Ovid in the episode "Under the Surface"
- I, Claudius (1976) – Agrippa Postumus
- The New Avengers (1976) – Colonel Miller in the episode "Dirtier by the dozen"
- The Three Hostages (1977) – Dominick Medina
- 1990 (1977) – Philip Carter
- The Prime of Miss Jean Brodie (1978) – Teddy Lloyd
- Lillie (1978) – Prince Louis of Battenberg
- The Professionals (1978) – Peter Crabbe / Tommy, CI5 Agent
- Strangers (1982) – Martin Hargreaves
- Reilly, Ace of Spies (1983) – Count Massino
- Miss Marple Series 1 Episode 1: "A Murder is Announced" (1985) – Detective Inspector Craddock
- The Crucifer of Blood (1991) – Neville St Clair
- Inspector Morse Series 5 Episode 3: "Who Killed Harry Field?" (1991) – Tony Doyle
- Miss Marple (TV Series) "The Mirror Crack'd from Side to Side" (1992) – Detective Inspector Craddock
- Tales of the Unexpected
- Pie in the Sky (1996) – Charles Rider
- Agatha Christie's Poirot Season 8 Episode 2: "Lord Edgware Dies" (2000) – Lord Edgware
- Lost Empires (with Laurence Olivier and Colin Firth)
- Lovejoy' (1994) Series 6, Episode 2: Day of Reckoning – Max Hunter
- The Ruth Rendell Mysteries (1997) – Mark in the episode "A Dark Blue Perfume"
- Nova (2000, Public Broadcasting Service) – David Irving
- Casualty (2004) – Brian 'Bullet' Taylor
- The Adventures of Sherlock Holmes
- The Princes in The Tower (2005, Channel 4 production) – Dr John Argentine
- Spooks (2006) – Jocelyn Myers
- Midsomer murders Series 10 Episode 3: King's Crystal (2007) – Charles King
- The Tractate Middoth (2013, BBC) – John Eldred
- Who's Who in the Theatre, 17th ed, 1981