Erant eiusmodi fere situs oppidorum ut posita in extremis lingulis promunturiisque neque pedibus aditum haberent, cum ex alto se aestus incitavisset, quod accidit semper horarum XII spatio, neque navibus, quod rursus minuente aestu naves in vadis afflictarentur. ita utraque re oppidorum oppugnatio impediebatur. ac si quando magnitudine operis forte superati, extruso mari aggere ac molibus atque his oppidi moenibus adaequatis, suis fortunis desperare coeperant, magno numero navium appulso, cuius rei summam facultatem habebant, omnia sua deportabant seque in proxima oppida recipiebant: ibi se rursus isdem opportunitatibus loci defendebant. haec eo facilius magnam partem aestatis faciebant quod nostrae naves tempestatibus detinebantur summaque erat vasto atque aperto mari, magnis aestibus, raris ac prope nullis portibus difficultas navigandi.
(Caesar, De Bello Gallico 3.12)
The positions of the strongholds were generally of one kind. They were set at the end of tongues and promontories, so as to allow no approach on foot, when the tide had rushed in from the sea—which regularly happens every twelve hours—nor in ships, because when the tide ebbed again the ships would be damaged in shoal water. Both circumstances, therefore, hindered the assault of the strongholds; and, whenever the natives were in fact overcome by huge siege-works—that is to say, when the sea had been set back by a massive mole built up level to the town-walls—and so began to despair of their fortunes, they would bring close in shore a large number of ships, of which they possessed an unlimited supply, and take off all their stuff and retire to the nearest strongholds, there to defend themselves again with the same advantages of position. They pursued these tactics for a great part of the summer the more easily because our own ships were detained by foul weather, and because the difficulty of navigation on a vast and open sea, with strong tides and few—nay, scarcely any—harbours, was extreme. (tr. Henry John Edwards)