Dead, mean millionaire Kovic (William Thourlby) invites a group of people (among them Scott Brady and a very drunk Virginia Mayo) who have every reason to hate him onto his private island to take part in the reading of his will.
Kovic's enemies believe the dead man wants to make amends by giving them a part of his fortune, but in truth, Kovic built various nice electronic tricks and traps in his house before he died - well, before he was killed by his housekeeper Lupe (pronounced "loopy" and played by Shelley Morrison), actually - because he believed one of his guests responsible for his near death some time ago, and wanted revenge for that even after his actual death. Come to think of it, Kovic might have been a bit mad.
Oh, and Kovic also built a killer robot wearing his own face. Not surprisingly, said killer robot soon begins killing. Who will be the last people standing?
Yes, people were still making Dark Old House movies in 1966, and the sub-genre was even still able to produce rather entertaining films. At least, that's how a film like Castle of Evil looks to my eyes today. I suspect the film's contemporaries might have experienced it as a rather bland, horribly old-fashioned piece of filmmaking that couldn't hold a candle to Corman's gothics or the output of Hammer of the time, what with its cast of bland middle-aged people (and Virginia Mayo having fun) playing bland middle-aged characters on bland sets for a mediocre director.
Looked at with a bit of temporal distance, Castle of Evil does not become a more timely movie, but it turns out to be pretty alright if you like Dark Old House movies in general. A lot of the film consists of it just going through the sub-genre motions. Fortunately, these motions are perfectly fine when experienced by an audience in the appropriate mood - too lazy to think much, too tired to think, that sort of thing.
Castle of Evil's only major divergence from its genre roots is also its one major merit (and comes as quite a surprise in a film that otherwise shows so little ambition to be different or interesting). Where your typical Dark Old House movie explains its seemingly supernatural happenings away with ridiculously contrived "natural explanations" and a guy in a gorilla costume, Castle decides that it's much better to explain away its own seemingly supernatural happenings with mad science and an evil android that should by all rights be fought by a masked wrestler. I, for one, approve of this decision, as I do of the film's other big decision to let Scott Brady destroy the android with a stationary laser gun thingie. This surprising bit of creativity doesn't exactly make up for all those scenes of bland, middle-aged drama, but it's just the sort of thing to make me remember Lyon's film fondly in the future when I'll have forgotten all if its flaws.