Thucydides, History of the Peloponnesian War
Benjamin Jowett, Ed.

4. The Syracusans and their allies now¹ began to build² a single line of wall starting from the³ city and running upwards across Epipolae at an angle with the Athenian wall; this was a work which, unless it could be stopped by the Athenians, would make the investment of the city impossible. [2] Towards the sea the Athenian wall was now completed, and their forces had come up to the high ground. Gylippus, knowing that a part of the wall was weak, instantly went by night with his army to attack it. [3] But the Athenians, who happened to be passing the night outside the walls, perceived this movement and marched to oppose him; whereupon he at once withdrew. They then raised the weak portion of their wall higher; and guarded it themselves, while they posted the allies on the other parts of the fortification in the places severally assigned to them. [4] Nicias now determined to fortify Plemmyrium, a promontory which runs out opposite the⁴ city and narrows the entrance to the Great Harbour. He thought that this measure would facilitate the introduction of supplies⁵. His forces would then be able to watch the harbour of the Syracusans from a nearer point, whereas they had hitherto been obliged to put out from the further corner of the Great Harbour whenever a Syracusan ship threatened to move. He was inclined to pay more attention than hitherto to naval operations; for since the arrival of Gylippus the Athenian prospects by land were not so encouraging. [5] Having therefore transferred his ships and a portion of his army to Plemmyrium, he built three forts in which the greater part of the Athenian stores were deposited; and the large boats as well as the ships of war were now anchored at this spot. [6] The removal was a first and main cause of the deterioration of the crews. For when the sailors went out to procure forage and water, of which there was little, and that only to be obtained from a distance, they were constantly cut off by the Syracusan cavalry, who were masters of the country, a third part of their force having been posted in a village at the Olympieum⁶ expressly in order to prevent the enemy at Plemmyrium from coming out and doing mischief. [7] About this time Nicias was informed that the rest of the Corinthian fleet⁷ was on the point of arriving, and he sent twenty ships, which were ordered to lie in wait for them about Locri and Rhegium and the approach to Sicily.