The Athenians then completed the building of their Circle, and began to lay the materials for the northern line of wall. By the advice of Hermocrates the Syracusans made no further attempt to attack them in full force, but began to build a counterwall, running out from the city in a direction south of the Athenian Circle, so as to cross the line to be followed by the wall of blockade, and prevent it from reaching the Great Harbour. The work proceeded without interruption, for the Athenians were engaged in their building operations north of the Circle, and did not choose to divide their forces. When it was completed, this counterwork consisted of a solid stone wall, crowned with wooden towers, and defended in front by a palisade. The blockade of Syracuse was thus rendered impossible, as long as the defenders could keep possession of their counterwall. But unfortunately the guards left in charge of the new wail soon began to neglect their duty, and erected tents in the shade, where they passed the hot hours of the afternoon, while some even left their posts, and went off to refresh themselves in the city. The Athenian generals did not fail to take advantage of this negligence. Watching their opportunity, when most of the Syracusan guards were reposing under the shelter of the tents, they sent a chosen troop of some three hundred men to make a sudden assault on the counterwall. Then, having divided the main body of the Athenian army between them, they disposed their forces so as to prevent any rescue from the town. One division was drawn up before the principal gate in the new Syracusan wall, while the other proceeded to a postern-gate, at the point where the counterwall started from the city. The combined movement was completely successful; the three hundred carried the stockade and cross-wall by storm, and compelled the defenders to take refuge within the ramparts of Syracuse. The whole Athenian army then marched up to the counterwall and stockade, which they speedily demolished, carrying off the materials for their own use. Wishing to prevent any second attempt on the part of the Syracusans to cut them off from the southern slope of Epipolae, the Athenian generals now fortified that part of the cliff which looks towards the Great Harbour. By occupying this point they obtained a new centre, commanding the space between the Circle and the southern edge of the cliff, and placing them in communication with the level valley of the Anapus, across which they had to carry their line of blockade. For the present building operations were suspended on the northern side of the Circle, as they wished first of all to complete the investment of Syracuse towards the south.