Together with the Rhine, the Danube was the northern boundary of the Roman Empire. Springing from two wells in the Black Forest and emptying itself into the Black Sea, it has a length of 2860 kilometers and is the largest river in Europe. All in all, ten legions guarded the border. As Procopius was to declare in the sixth century:
The Emperors of former times, by way of preventing the crossing of the Danube by the barbarians who live on the other side, occupied the entire bank of this river with strongholds, and not the right bank of the stream alone, for in some parts of it they built towns and fortresses on its other bank.