Well, at the 52 B.C. Roman conquering of the Paris basin, it was already an important crossroads between river and road travel (a place where a major north-south route crossed the Seine river across its central island), but it is not certain that the area was the major habitation then (the nearest known major Celtic population centre was in today's Sens). Anyway, the Romans took an interest Paris' island it for its strategic position for a garrison and lightly fortified it, but when it later become a trading centre, Gallo-Roman growth spread to the Left Bank.