Quotation from: The Gods of Mars

Written by: Edgar Rice Burroughs

Then I had him draw a map of Omean, indicating plainly the position
of Shador and of the opening in the dome which led to the outer
world.

These I studied until they were indelibly imprinted in my memory.
From Xodar I learned the duties and customs of the guards who
patrolled Shador. It seemed that during the hours set aside for
sleep only one man was on duty at a time. He paced a beat that
passed around the prison, at a distance of about a hundred feet
from the building.