 Constantine 's share of the Empire consisted of Britain , Gaul , and Spain . He therefore commanded one of the largest Roman armies , stationed along the important Rhine frontier . After his promotion to emperor , Constantine remained in Britain , driving back the tribes of the Picts and secured his control in the northwestern dioceses . He completed the reconstruction of military bases begun under his father 's rule , and ordered the repair of the region 's roadways . He soon left for Augusta Treverorum ( Trier ) in Gaul , the Tetrarchic capital of the northwestern Roman Empire . The Franks , after learning of Constantine 's acclamation , invaded Gaul across the lower Rhine over the winter of 306 – 307 AD . Constantine drove them back beyond the Rhine and captured two of their kings , Ascaric and Merogaisus . The kings and their soldiers were fed to the beasts of Trier 's amphitheater in the adventus ( arrival ) celebrations that followed . 
