The castle is built into the side of a cliff, which otherwise would seem like an indefensible position with easy attack points from above. In this case, though, the wall above Predjama is cantilevered out enough that the structure is almost entirely overhung by rock. This accounts for the narrow shape of the structure; in order to protect the walls from missiles launched from the top of the cliff, it was necessary to make the contour of the wall fit beneath the hollow. Interestingly, the roofs of Predjama rarely get wet because it is so well protected by the overhang.
The forward defenses are fairly standard, with high, strategically elevated walls and only one narrow point of entrance. Behind Predjama, though, is an entrance to the huge Postojna cave system, which stretches for miles through the karst of this region. A well guarded, little known passageway extends from the castle some fifteen miles to Vipava, a vineyard town to the north, and a source of much needed supplies. In 1483, the knight Erazem Lueger was besieged in the castle after he killed a relative of the Austrian emperor in a duel. The siege lasted for over a year, and the Austrian army was confounded by the fact that Erazem was able to supply his defending forces with fresh food for so long. At one point, in the spring of 1484, the knight sent a barrage of fresh cherries over the wall to taunt his attackers.
Another strategic advantage of the cave is the constant supply of fresh water that seeps from the stalactites in the rear chambers. The near constant flow made a well unnecessary and forced modern keepers of the castle to build a drainage system (visible above) to catch and remove the water from the cavern.
The original detachment bridge that spanned the gap to the approach path has long since been stoned up and made permanent, but the old chasm between castle and solid ground would have been very difficult to cross. In the later stages of the castle's defensive history, it became increasingly vulnerable to gunpowder ballistics, which could be arranged very easily across the valley from the walls. It's extraordinary that Predjama was able to survive mostly intact until the present, but that has more to do with diplomacy and obsolescence than modern effectiveness.
An overly-satisfying tavern with heavy food and a castle theme sits close by to the walls - our lunch was filling and presented to us on wooden boards, which was charming. It can be difficult to reach Predjama without one's own car - there are buses but they are sporadic and require many changes and a long walk. Despite being one of Slovenia's major tourist sights, the castle remains only somewhat touristy, with a smattering of visitors and a very rural setting. It's very much worth seeing though - this is a castle unlike any other that I know of, with more quirks and corners than many larger, more visited piles.