The siege of Plataea had now lasted for more than a year, and the brave garrison began to be in sore straits, for their supplies were giving out, and they had no hope of rescue from outside. In this desperate situation they resolved to make an attempt to break through the besieging lines, and make their escape to Athens. All were to take part in the adventure, leaving the Peloponnesians in possession of an empty town. But when the time came for carrying out this bold design, half of the garrison drew back, thinking the risk too great. The other half, numbering about two hundred and twenty, persisted in their purpose, and forthwith fell to work on their preparations. They began by making ladders for scaling the enemy's wall; and in order to ascertain the proper length of the ladders, they counted the courses of bricks in a part of the wall facing the town, which happened to have been left unplastered. Many counted the courses together, and by repeating the process over and over again, and comparing the result, they at last hit upon the right number. When once this was known, they could easily calculate the length of their ladders, for the bricks were all of the same dimensions, and they knew the thickness of a single brick. The Peloponnesians had built a double line of wall round Plataea, the two lines being separated by a distance of sixteen feet. The whole of the space within this double wall was covered by a flat roof, so as to present the appearance of a single thick wall, with battlements on either side; and this covered space, which was divided into rooms by partition-walls, served as barracks for the besiegers. Along the top were high towers, with intervals of ten battlements between them, and built flush with the wall on both sides, so as to leave no passage, except through the middle of the tower. These served as guard-rooms, where the soldiers on duty took shelter on wet and stormy nights. For the distance between the towers was very small, and they could rush out and man the walls at a moment's notice.