C. Julius Caesar, Gallic War

The sites of their towns were generally such that, being placed on extreme points [of land] and on promontories, they neither had an approach by land when the tide had rushed in from the main ocean, which always happens twice in the space of twelve hours; nor by ships, because, upon the tide ebbing again, the ships were likely to be dashed upon the shoals. Thus, by either circumstance, was the storming of their towns rendered difficult; and if at any time perchance the Veneti overpowered by the greatness of our works, (the sea having been excluded by a mound and large dams, and the latter being made almost equal in height to the walls of the town) had begun to despair of their fortunes; bringing up a large number of ships, of which they had a very great quantity, they carried off all their property and betook themselves to the nearest towns; there they again defended themselves by the same advantages of situation. They did this the more easily during a great part of the summer, because our ships were kept back by storms, and the difficulty of sailing was very great in that vast and open sea, with its strong tides and its harbors far apart and exceedingly few in number.