Caesar - Gallic War - Book I, 38 - Vesontio (Besançon)
He had advanced a three days' march when news was brought to him that Ariovistus with all his forces was pushing on to seize Vesontio (Besançon), the largest town of the Sequani, and had already advanced a three days' march from his own borders. Caesar considered that a great effort on his part was needed to prevent this occurrence. For there was in that town an abundant supply of all things needful for war, and the place was so well fortified by Nature as to afford great facilities for the conduct of a campaign. The river Dubis (Doubs), with a circuit that might have been traced by compasses, surrounds well-nigh the whole town: the remaining space of not more than sixteen hundred feet, where the river breaks off, is closed in by a height of great eminence, so placed that its foundations touch the riverbank on either side. This height is surrounded by a wall to form a citadel and join it with the town. It was to this place that Caesar pushed on with forced marches by night and day, and, seizing the town, posted a garrison in it.