The stairway between the Second and Third Gates was shorter than the stairway between the First and Second.
He noticed this after the first few minutes of climbing and sat with the observation without drawing conclusions from it immediately. The mountain's architecture served the mountain's purposes, not the convenience of anyone ascending through it, and the purposes were not always apparent from inside the structure they expressed. Shorter might mean the Third Gate was closer to the Second because the threshold between them was smaller. Or it might mean the transition zone was more compressed because the pressure within it was higher and a longer transition would be more than the passage required.
He found, as he climbed, that it was the second.
