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.