A cliffside Slovenian castle sits on not just a network of secret caves that hid an infamously swashbuckling knight—who successfully held off a full offensive by the Holy Roman Empire for more than a year—but also a cache of buried treasure.

Fitted into the mouth of a cavern in the eastern European countryside, Predjama Castle appears to be part of the mountain in which it’s built.

The castle’s history stretches back to at least 1202, when it was first mentioned in writing. Over the next 200 years, it belonged to a group of Austrian dukes, a fleet of knights, and later, a “rebel knight” called Erazem Lueger.