Predjama Castle was built into a natural cave system, so is quite unique. The legend involves a knight, Erazem, in the 15th century.
The door (below left) is in the castle courtroom. If decisions were not favourable, the door was opened and the person pushed through it (a 60m drop onto the rocks below). Below right shows the family room - the only room with heating at the time of Erazem. All other rooms for soldiers and guests were unheated and, being built into a cave, were very cold.
View from the castle to the surrounding area
Soldiers quarters - now made into a display.
Postojna Cave is about 10km from the castle - a fairly large cave that involves a 2km train ride into the cave, then a walk for about 1km to see some of the formations. This is only a small section of the cave though, as it 24km in total.
Some of the formations....On the picture below, you can see the magnitude of the cave in comparison to the people.