L. Frank Baum

“ The Flatheads use the stairs for coming down and going up. If there was a solid stone wall to keep them from reaching the plain they would themselves be imprisoned by the wall. So they had to leave some place to get around the wall, and, if the wall was visible, all strangers or enemies would find the place to go around it and then the wall would be useless. So the Flatheads cunningly made their wall invisible, believing that everyone who saw the entrance to the mountain would walk straight toward it, as we did, and find it impossible to go any farther. ”

“ Ozma was feeling around, her bands outstretched before her.
"Yes, dear, it is magic," she replied. "The Flatheads had to have a way from their mountain top from the plain below, but to prevent enemies from rushing up the stairs to conquer them, they have built, at a small distance before the entrance a wall of solid stone, the stones being held in place by cement, and then they made the wall invisible."
"I wonder why they did that?" mused Dorothy. "A wall would keep folks out anyhow, whether it could be seen or not, so there wasn't any use making it invisible. ”

“ Let's walk around a ways; perhaps we'll find a ladder or something."
They walked quite a distance, for it was a big mountain, and as they circled around it and came to the side that faced the palm trees, they suddenly discovered an entrance way cut out of the rock wall. This entrance was arched overhead and not very deep because it merely led to a short flight of stone stairs.
"Oh, we've found a way to the top at last," announced Ozma, and the two girls turned and walked straight toward the entrance. Suddenly they bumped against something and stood still, unable to proceed farther. ”