Predjama Castle holds the world record for largest castle in the world built into a cave. We don't do many tours like this when we travel, but this one was worth it. It's like something out of a fairy tale, with a fascinating history dating back to the 13th century. We recommend arriving at the beginning of the day or at the end of the day to enjoy the castle with less crowds. We'd also recommend buying your tickets in advance during high season, and don't skip the audio guide!