And prob'bly anybody that tries to go up the stairs gets bumped, as we did." Ozma made no reply at once.

Her face was grave and thoughtful.
"I think I know the reason for making the wall invisible," she said after a while.

"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.