Very old black and white film watched as a kid

A scifi/fantasy film. The film starts with a couple or group of friends arriving at some kind of mansion/old spooky hotel on a cliff above an ocean. During the night a secret door opens and a couple or the whole group are kidnapped by some unusual bipedal sea creatures with tridents. They are taken through the secret door down a seaweed encrusted cavernous staircase, and taken to an underwater kingdom. Its kind of got a Captain Nemo vibe may even be a nemo film. I really want to watch it again!
Im 52 this was a very old black and white film.

Sigmund and the sea monsters!

“War-Gods of the Deep” (1965), also known as “City in the Sea”?

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