Predjama Castle is a medieval fortress built into a massive cave mouth 123 meters high in a vertical cliff, making it one of the world's most dramatic castles. Constructed in the 13th century, it became famous for 15th-century robber baron Erasmus Lueger who resisted a year-long siege by the Austrian Emperor using a secret tunnel for supplies. The castle features several floors of furnished rooms, a museum, and secret passages. The cave beneath the castle is open only in summer (June-August) due to bat hibernation. Combined with Postojna Cave, it makes for a perfect day trip exploring Slovenia's unique karst heritage.