Compare the entrance to Surprise Valley under the stone bridge and the entrance to the ancient cave dwellers up the steep steps. How are these the same and how are they different?

The entrance to Surprise Valley and the entrance to the ancient cave dwellers are both up steep cliffs. This was to protect the occupant as they could see out and down for anyone trying to attack their homes. They are similar in that they each have steps carved into the side of the cliffs, but they are different because Balancing Rock gives the entrance to Surprise Valley an extra protection. Maybe the ancient cliff dwellers never pushed the rock because they were afraid to be locked in the valley forever and this may be important to Venters and Bess, knowing that the cliff dwellers are no longer there.