History of the Peloponnesian War

And the Athenians by this time, having made an end of their wall to the sea, were come up again; and Gylippus (for some part of the wall was but weak), rising with his army by night, went to assault it.

But the Athenians, also knowing it (for they lodged all night without the wall), went presently to relieve it; which Gylippus perceiving, again retired. And the Athenians, when they had built it higher, kept the watch in this part themselves, and divided the rest of the wall to the charge of their confederates. Also it seemed good to Nicias to fortify the place called Plemmyrium.

It is a promontory over against the city, which, shooting into the entrance of the great haven, straiteneth the mouth of the same; which fortified, he thought would facilitate the bringing in of necessaries to the army. For by this means, their galleys might ride nearer to the haven of the Syracusians, and not upon every motion of the navy of the enemies to be to come out against them, as they were before, from the bottom of the [great] haven. And he had his mind set chiefly now upon the war by sea, seeing his hopes by land diminished since the arrival of Gylippus.