The Peloponnesian fleet was the first to arrive, and the Spartans, who were now present in full force with their allies, determined to make the most of their time. They hoped, by a simultaneous onslaught of their army and fleet, to carry the fort before the Athenian ships had time to return. But in case they should fail in this, they intended to cripple the movements of the relieving squadron, by blocking the entrances to the bay. For the long, narrow island of Sphacteria forms a natural break water, converting the harbour of Navarino into a land-locked basin, with two narrow passages at the northern and southern end. [Footnote: The description of Thucydides does not correspond to the picture of the harbour given in our modern maps. But in the course of twenty centuries great changes may well have occurred.] These inlets the Spartans proposed to close, by anchoring triremes close together, with their prows turned seawards, which they could easily have done, for at the southern entrance there was only room for eight or nine vessels to sail abreast, and at the northern entrance only room for two. This precaution, however, was never carried out; and the Spartans, as if blinded by fate, adopted another measure, which led to fatal consequences for themselves. Wishing to keep command of every spot of land in the neighbourhood of Pylos, they landed a body of their own men, numbering four hundred and twenty, with the usual proportion of Helots, on the island, and the same time posted troops at every assailable point on the opposite coast. Thinking now that the little garrison at Pylos, surrounded on all sides by enemies, would fall an easy prey, they sent orders to the fleet to get under way, and prepared to attack the fort on the land side. Meanwhile Demosthenes had not been idle: having drawn his three remaining ships under the shelter of the fort, and protected them in front by a stockade, he armed the crews with such weapons as he had, including a number of wicker-shields, taken from a thirty-oared Messenian galley which had recently come to his assistance with a force of forty hoplites. Then, having posted the greater part of his troops for the defence of his position against the Peloponnesian army, he himself descended with a picked body of sixty hoplites, and took up his station on the rocky shore. For on this side the defences were weakest, as the Athenians, in building the fort, had never anticipated an attack from the sea.