The automobile that Steed and Mrs. Peel drive into the water at the end is an Amphicar Model 770, produced between 1961-1968.
Allington Castle, a stone-built moated castle built in the late 13th century in Allington, Kent, England was used for the filming of Castle De'ath.
The clip of cascading water when the submarine pen is flooding is taken from the film The Dambusters when the dam is breached.
On their first evening in Castle De'ath, just before bedtime, "McSteed" excuses himself with the line "The flowers of the forest are all wede away." "What?" asks Mrs Peel. "Must be Robby Burns," responds McSteed. (wede awa' = carried off, as by death; withered) Various websites note that Jean Elliot actually wrote this in 1756-1758, reworking older lyrics. "Flowers of the Forest" as a tune was written as a lament for the 10,000 Scots killed at the Battle of Flodden in 1513. Robert Burns, in a 1793 letter, additionally credited a Mrs. Cockburn, "authoress of a beautiful variation of 'The Flowers of the Forest'".
Until the very last scene, Patrick Macnee never wears suit or trousers in this episode - he is kilted throughout and wears a cardigan.