Predjama Castle, Slovenia
Built into the side of a cave, Predjama Castle in Slovenia is without doubt one of the most impressive mountainside buildings in the world. The castle dates all the way back to 1274 but had to be rebuilt in 1570 after the original building was destroyed in a siege. The rear of the castle is also just as impressive as the front, as a passageway cuts through the rock and leads to a secret opening at the top of the mountain!
