The weakest point in the chain of defence was the Northern corner of the Adriatic, and the increasing prosperity of the great plains of the Po after they had become a Roman province naturally attracted the attention of the semi-civilized tribes who lived in the hills along the Dalmatian coast. Not only was there danger from the East, but the valley of the Adige formed a gateway through which Central Europe could pour its restless multitudes upon the Cis-Alpine Province. The geographical configuration of the regions south of the plains of the Eastern Danube has always impeded their progress, and to this very day a patch of backward nationalities remains there in close proximity to the most elaborately civilized states of Europe. The other weak spot was the course of the Rhine, and especially the country below the Drakensberg; that noble river for many miles from the Lake of Constance formed a natural defence against the Germanic hordes, but on reaching the flat land below Cologne it spread into marshes and split into smaller channels, in which flotillas of boats could be prepared without attracting notice, as was necessarily the case where the river ran in a single stream. In fact it was practically found that in places the Rhine was no barrier, and that the tribes on its Eastern bank must be rolled back from the river, if Gaul was to enjoy her new prosperity in peace. It was in the defence of these two weak spots that Tiberius was to fight his chief campaigns. In both regions security demanded that the operations should be conducted far beyond the frontier, in country difficult at the present day, and tenfold more difficult then, when extensive forests and marshes were added to the impediments offered by ravines and mountains.