Caesar does not say what the cause of the conflict was, but the Sequani controlled access to the Rhine river along the valley of the Doubs. To that end, they had gradually built up an oppidum, or fortified town, at Vesontio. Tradesmen headed up the Rhone and its tributary the Saône (the ancient Arar) could not pass the Doubs at Vesontio without coming to terms with the Sequani, and no one could pass from the Rhine to the Rhone but on similar terms. The east of the entire great channel is bordered by the Jura mountains and the west by the Massif Central. Vesontio is from that stretch of the Rhine between Mulhouse and Basel.