Predjama Castle is a spectacular Renaissance fortress built dramatically into the mouth of a cave in a sheer cliff face, near Postojna. Ticketed visitors can explore its rooms, passages and the secret tunnels behind, learning the legend of the robber baron Erazem who once withstood a long siege here. Half-hidden in the rock and surrounded by wild karst scenery, it is one of the most striking castles in Europe and a natural pairing with a visit to the nearby Postojna Cave.