Caesar - Gallic War - Book III, 12 - Location and structure of the oppida
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 inshore 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.
Description of the oppida
Map of the oppida