It was almost exactly the same width as the stairway, so that when it stood wide open you could not have put your hand between its edge and the stairway wall.

Lying on the floor of the hall within a few feet of the nearest corner was a length of good sound olive-wood, about three inches in thickness, roughly squared and not particularly squared.

Having stepped on it accidentally, I picked it up, and discovered more by accident than intention that it was longer than the width of the stairway.

Then I noticed a notch in the stairway wall.

Behind the opened door there was a deeper notch in the opposite wall.