Roger Corman decided to do an H.P. Lovecraft story as a break from his Edgar Allan Poe series while keeping the elements that made it successful. American-International took no chances. It gave the film a "Poe" title and marketed it as another in the series.
The footage scene of the castle interior is similar to the scene in The Raven (1963) movie, which was directed by Roger Corman as well. Also the shots of castle from outside under thunder and lightning, and waves breaking of rocks appear in both movies.
Although the American Film Institute Catalog of Feature Films 1961-1970 identifies I. Stanford Jolley as the Boat Captain [sic], IMDb correctly lists him as Carmody, the Coachman; no Boat Captain appears in the film.