The headquarters of the Rhine legions continued to be at Mainz and Xanten with summer quarters at the new Colonia which became Cologne. Four legions of the Upper Army were stationed at the former, and four of the Lower Army at the latter. In due course, we cannot say when, these became the centres of two separate provinces. On the Danube there were three legions in Pannonia, the great new Austrian province. Along this frontier there was now a double line of Cæsarian provinces. Rhætia and Noricum were conquered in 15 B.C. Then there were tedious and unprofitable campaigns in the southern Swiss valleys as the result of which a row of little Alpine prefectures was established. There is still a fine monument to Augustus on the heights above Monaco enumerating forty-six Alpine tribes made subject to Rome. It was erected by the gratitude of the Italian farmers, for the Alpine tribes had always scourged the plains. Roads were constructed here and there over the Alps. The principal pass to Germany lay by way of Turin and the St. Bernard with Augusta (Aosta) to guard it. In Pannonia the old route from Aquilegia over the Julian Alps was restored and a new _Via Claudia_ constructed up the valley of the Adige from Tridentum (Trent) to Augusta (Augsburg). To round off the Danube frontier Mœsia or Mysia was conquered quite at the beginning of the period and added as an Imperial province, probably in A.D. 6, under a prefect. It stretched along the south bank of the Danube, down to the Black Sea, and embraced part of the Balkan high lands. Thus with strong legions posted in permanent encampments all along the Rhine and Danube, Rome had now a satisfactory northern frontier which only required guarding to keep Rome and Italy in security.