Looking for title of an old TV movie.

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I’m looking for the title of an old TV horror/sci-fi movie (80’s?) about a small group of people picnicking in a rural area who stumble on a door to an alternate universe. Their initial excitement turns to horror as they are knocked off one by one. The only thing I remember about the strange place is that there might have been a castle there. The only survivor is locked up in a psych hospital, and I believe that the film ends with one of the people from the bad universe arriving there to finish her off.

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Sally Selected answer as best Jun 14, 2022
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Equinox (1970).
https://www.imdb.com/title/tt0067055/

Google: movie|film+picnic+universe|dimension+horror|thriller+castle

Sally Selected answer as best Jun 14, 2022

That’s it-thank you so much!