 In 356 during his first campaign he led an army to the Rhine, engaged the inhabitants there and won back several towns that had fallen into Frankish hands, including Colonia Agrippina (Cologne).With success under his belt he withdrew for the winter to Gaul, distributing his forces to protect various towns, and choosing the small town of Senon near Verdun to await the spring.This turned out to be a tactical error, for he was left with insufficient forces to defend himself when a large contingent of Franks besieged the town and Julian was virtually held captive there for several months, until his general Marcellus deigned to lift the siege.