History of the Peloponnesian War
Thucydides
Thucydides. History of the Peloponnesian War. Crawley, Richard, translator. London and Toronto: J. M. Dent and Sons Ltd.; New York: E. P. Dutton and Co., 1914.
afterwards, half hung back, thinking the risk great; about two hundred and twenty, however, voluntarily persevered in the attempt, which was carried out in the following way.
Ladders were made to match the height of the enemy's wall, which they measured by the layers of bricks, the side turned towards them not being thoroughly whitewashed. These were counted by many persons at once; and though some might miss the right calculation, most would hit upon it, particularly as they counted over and over again, and were no great way from the wall, but could see it easily enough for their purpose.
The length required for the ladders was thus obtained, being calculated from the breadth of the brick.
Now the wall of the Peloponnesians was constructed as follows. It consisted of two lines drawn round the place, one against the Plataeans, the other against any attack on the outside from Athens, about sixteen feet apart.
The intermediate space of sixteen feet was occupied by huts portioned out among the soldiers on guard, and built in one block, so as to give the appearance of a single thick wall with battlements on either side.