B-movie Mayhem...The Terror, Corman-Poe cycle honourable mention...

First, here is the truly wacky synopsis (with spoilers) from Wikipedia: In 1806, Andre Duvalier, a French soldier lost in the Confederation of the Rhine, is saved by Helene, a young woman who bears a resemblance to Ilsa von Leppe, the wife of Baron von Leppe who died 20 years before. Andre sets out to investigate Helene's true identity and, in doing so, learns the Baron's darkest secret: after he found Ilsa with another man, the Baron killed his wife while his servant killed her lover.
Over the last two years, the Baron has been tormented by Ilsa's ghost, who has beseeched him to kill himself so that they can be together forever. After much hesitation, the Baron decides to do so and atone for his crimes. Unbeknownst to him, Ilsa's ghost is being commanded to haunt him by a peasant witch named Katrina.
After preventing the Baron from killing himself, Andre and Stefan, the Baron's major domo, capture Katrina and force her into compliance. Katrina then reveals herself to be the mother of a man named Eric, whom she believes that the Baron killed 20 years before and hopes to avenge his death by damning his soul to Hell. In a stunning revelation, Stefan reveals that Eric was Ilsa's lover, that it was the Baron who had died 20 years ago, not Eric, and that Eric felt so guilty about it that he took the Baron's place, pretending to be him (thus explaining why the Baron had never left the castle in the past 20 years). Over the years, Eric has convinced himself that he is the true Baron von Leppe.
Realizing her error too late, Katrina goes with Andre and Stefan to stop Eric from flooding the castle crypt. Katrina's pact with the devil, however, makes her unable to walk on consecrated ground and she ends up burning to death after being struck by lightning.
At the von Leppe castle, Eric floods the crypt as Ilsa's ghost attempts to kill him and Stefan struggles to stop her. By the time Andre gains access to the crypt, it is already starting to cave in and he is only able to save Helene. The two share a moment outside the castle before Helene turns into a rotting corpse.

• Boris Karloff as Baron von Leppe/Eric, a corrupt aristocratic man who murdered the baron and his wife and poses as the baron of an abandoned castle.
• Jack Nicholson as Andre Duvalier, a soldier of Napoleon's army who finds himself lost after fleeing his men during battle.
• Dick Miller as Stefan, the Baron's son and trustful servant who serves the impostor.
• Sandra Knight as Helene/Ilsa, the shapeshifting demon of the witch who poses as the baron's deceased wife.
• Dorothy Neumann as Katrina the witch, a peasant woman and the mother of Eric who was driven out of the village for heresy and witchcraft.
• Jonathan Haze as Gustaf, a lost village man who became the mentally ill servant of the witch.
  

The Terror Trivia:

• Roger Corman made this picture while shooting his cycle of Edgar Allan Poe adaptations, specifically, right after The Raven was shot, and using the same sets. Corman considers it an "honorary member" of the Poe cycle (and therefore, so do I).
  
• A number of the previous Poe cycle pictures had ended with the castle or mansion being destroyed by fire (created with the same footage I previously described in my Fall of the House of Usher post -- "...the old barn which is used as some of the mansion-burning footage at the end of the film, and subsequently as stock footage in many of the other Corman-Poe cycle films, was located in Orange County and slated to be demolished, so Corman was able to strike a deal that allowed him to burn it and film it one night."), but this time Corman decided to change the formula and have the castle destroyed by a flood instead.
  
• Roger Corman shot most of film in four days (and these four days were the only ones which included Boris Karloff), but the second-unit work was filmed over a nine month period by five directors: Francis Ford Coppola, Dennis Jakob, Monte Hellman, Jack Nicholson, and Jack Hill. Jack Nicholson directed the climactic flood sequence. Francis Ford Coppola took eleven days to shoot his  footage. Only ten minutes ended up in the finished film.
  
• Both the front of the palace and the underground dungeon from The Haunted Palace later appeared in The Terror, which was shot on sets from other AIP films.
  
• Jack Nicholson claims to have nearly drowned while filming in Big Sur.
  
• Sandra Knight, who was married to Jack Nicholson at the time, was pregnant while filming (with daughter, actress Jennifer Nicholson).