Eugène Lourié had two models of the gantry built. The second was made of balsa wood for the explosion scene. This insured complete destruction of the gantry while avoiding the danger of the set being sprayed by shrapnel.
One of eight movies Dana Andrews appeared in during the year 1965.
One scene briefly shows the back of one of the Land Rovers showing the Land Rover Santana badge. These were built under license in Spain, which fits with the Spanish filming locations.
Both Dana Andrews and Janette Scott are mentioned in The Rocky Horror Picture Show song Science Fiction, which is a reference-laden homage to b-movies. Dana for Curse of the Demon and Janette for Day of the Triffids. Ironically, Janette's costar in Triffids is her co-star in this movie, Kieron Moore. Also, during this same year, 1965, Andrews co-starred with Anne Francis in two different movies, and Francis is also mentioned in the song Science-Fiction (for Forbidden Planet).
Dana Andrews has starred with Janette Scott's future husband, Mel Torme, in The Fearmakers (1958).