The Magic Springs Of Gerenia(1984)

Original title: Os Abismos da Meia-Noite

Irene is an agent for an insurance company. She is given the job of investigating the disappearance of an elderly librarian in a provincial town. During her investigations she meets a history teacher - Ricardo - who knew the librarian and is interested in local legends. Irene happens to hear about a very old medieval castle which, according to legend, has a secret entrance that opens mysteriously on Christmas night as the clock strikes twelve.