The siege of Plataea now began in earnest. First the town was surrounded with a palisade, to prevent anyone from escaping, the materials being taken from the plantations in the neighbourhood of the town. Then they raised a mound against the wall, expecting that with so large a force as theirs they would easily carry the place by storm. Timber was brought from Cithaeron, and with this they set up two stout buttresses of cross-beams, at right angles to the town-wall, to serve as a support on either side of the mound. Within this framework they piled up fascines, stones, earth, and whatever else was at hand. The whole army was employed in this task, which was continued for seventy days and nights without intermission, the men working in regular spells. Meanwhile the Plataeans had not been idle. First they built a wall of bricks and timber opposite to the point where the mound was rising, and resting on the ramparts, in order to raise the height of their defences. The new wall was covered with hides, raw and dressed, to protect the timber and the workmen from being injured by burning arrows. And while this structure was in progress, they made a breach in the old wall, and carted away the earth from the bottom of the mound. To prevent this, the Peloponnesians filled up the space thus caused with heavy masses of clay, rammed tightly into baskets of osier, which made a solid structure, much harder to remove than the loose earth. Then the Plataeans had recourse to another device: marking carefully the position of the mound, they ran a mine from the city under it, and as fast as the earth fell in, they carried it away. This continued for a long time, for the Peloponnesians, who saw their mound rising no higher, for all their labour, but rather growing less, did not guess the cause, but went on heaping up materials, which were swallowed up as fast as they were brought. Still the Plataeans feared that in spite of these counterworks they would at length be overpowered by numbers, unless they contrived some better means of defence. So they left off building the wall of bricks and timber, and beginning at either end of it, they built a crescent-shaped wall, curving inwards towards the city. Thus the Peloponnesians, if they succeeded in carrying the first wall, would find themselves confronted by a second line of defence, and would have all their work to do over again, besides being exposed to a cross-fire.