We may admit that Cæsar started from the neighbourhood of Nantes, and directed his march to the Roche-Bernard, where he crossed the Vilaine. Having arrived in the country of the Veneti, he resolved to profit by the time which must pass before the arrival of his fleet to obtain possession of the principal _oppida_ where the inhabitants took refuge. Most of these petty fortresses on the coast of the Veneti were situated at the extremities of tongues of land or promontories; at high tide they could not be reached by land, while at low tide the approach was inaccessible to ships, which remained dry on the flats; a double obstacle to a siege. The Romans attacked them in the following manner: they constructed on the land, at low tide, two parallel dykes, at the same time serving for terraces (_aggere ac molibus_), and forming approaches towards the place. During the course of construction, the space comprised between these two dykes continued to be inundated with water at every high tide; but as soon as they had succeeded in joining them up to the _oppidum_, this space, where the sea could no longer penetrate, remained finally dry, and then presented to the besiegers a sort of place of arms useful in the attack.[281] With the aid of these long and laborious works, in which the height of the dykes finished by equalling that of the walls, the Romans succeeded in taking several of these _oppida_. But all their labours were thrown away; for, as soon as the Veneti thought themselves no longer safe, they evacuated the _oppidum_, embarked with all their goods on board their numerous vessels, and withdrew to the neighbouring _oppida_, the situations of which offered the same advantages for a new resistance. The greater part of the fine season had passed away in this manner. Cæsar, convinced at length that the assistance of his ships was indispensable, came to the resolution of suspending these laborious and fruitless operations until the arrival of his fleet; and, that he might be near at hand to receive it, he encamped to the south of the Bay of Quiberon, near the coast, on the heights of Saint-Gildas. (_See Plate 12._) The vessels of the fleet, held back by contrary winds, had not yet been able to assemble at the mouth of the Loire. As the Veneti had foreseen, they navigated with difficulty on this vast sea, subject to high tides, and almost entirely unfurnished with ports. The inexperience of the sailors, and even the form of the ships, added to their difficulties.