History of the Peloponnesian War

Subsequently half of them shrank from it, thinking it a great risk; but about two hundred and twenty voluntarily persevered in the attempt, [which they effected] in the following manner.

They made ladders to suit the height of the enemy's wall, measuring by the layers of bricks, where the wall looking toward them happened to be imperfectly whitewashed. Now many counted the layers at the same time; and though some would naturally miss the correct calculation, the majority would attain it; especially as they counted them many times over, and were also at no great distance, but the wall was easily observed by them as far as they wished.

In this way they ascertained the proper length of the ladders, guessing the measure from the thickness of the bricks.

Now the wall of the Peloponnesians was of the following construction. It consisted of two lines round the place, one against the Plataeans, and another in case any one should attack them on the outside from Athens; and the lines were about sixteen feet apart.

In this interval then of the sixteen feet there were quarters built, and partitioned out amongst the soldiers that were on guard; and these were continuous, so that it appeared but one thick wall, with battlements on each side.