We climbed through the ivy until we found a clearing with this huge, wooden door. Imagine our surprise when we found it was completely unlocked.
Behind this door wasn't much, just two long, nondescript concrete hallways which didn't lead anywhere. But we had a real horror-movie moment when we emerged back out to the clearing, and found ourselves surrounded by identical high walls and with identical wooden doors, with no clue as to which direction we had come from.
We picked another door and entered another set of concrete passages.