Both the army and the fleet now began to occupy themselves seriously with the construction of the southernmost part of the wall of circumvallation; beginning immediately below the Athenian fortified point of descent from the southern cliff of Epipolæ, and stretching across the lower marshy ground to the Great Harbor. The distance between these two extreme points was about eight stadia or nearly an English mile: the wall was double, with gates, and probably towers, at suitable intervals, inclosing a space of considerable breadth, doubtless roofed over in part, since it served afterwards, with the help of the adjoining citadel on the cliff, as shelter and defence for the whole Athenian army. The Syracusans could not interrupt this process, nor could they undertake a new counter-wall up the mid-slope of Epipolæ, without coming out to fight a general battle, which they did not feel competent to do. Of course the Circle had now been put into condition to defy a second surprise. But not only were they thus compelled to look on without hindering the blockading wall towards the Harbor. It was now, for the first time, that they began to taste the real restraints and privations of a siege.[380] Down to this moment, their communication with the Anapus and the country beyond, as well as with all sides of the Great Harbor, had been open and unimpeded; whereas now, the arrival of the Athenian fleet, and the change of position of the Athenian army, had cut them off from both,[381] so that little or no fresh supplies of provision could reach them except at the hazard of capture from the hostile ships. On the side of Thapsus, where the northern cliff of Epipolæ affords only two or three practicable passages of ascent, they had before been blocked up by the Athenian army and fleet; and a portion of the fleet seems even now to have been left at Thapsus: so that nothing now remained open, except a portion, especially the northern portion, of the slope of Epipolæ. Of this outlet the besieged, especially their numerous cavalry, doubtless availed themselves, for the purpose of excursions and of bringing in supplies. But it was both longer and more circuitous for such purposes than the plain near the Great Harbor and the Helôrine road: moreover, it had to pass by the high and narrow pass of Euryâlus, and might thus be rendered unavailable to the besieged, whenever Nikias thought fit to occupy and fortify that position. Unfortunately for himself and his army, he omitted this easy but capital precaution, even at the moment when he must have known Gylippus to be approaching. [380] Thucyd. vi, 103. οἷα δὲ εἰκὸς ἀνθρώπων ἀπορούντων καὶ μᾶλλον ἢ πρὶν πολιορκουμένων, etc.