Even King Henry had found it necessary, before the end of his reign, to rely upon ecclesiastical support, especially in his efforts to civilise the marks. There the fortified churches and monasteries became, like the new walled towns, centres of defence, besides being the only homes of civilisation and culture in those wild regions. But King Henry had not removed the danger of Wendish invasion, and the civil wars of Otto’s early years gave a new opportunity for the heathen to ravage the German frontiers. In the midst of Otto’s worst distress, Hermann Billung kept the Wends at bay, and taught the Abotrites and Wagrians, of the lands between the lower Elbe and the Baltic, to feel the might of the German arms. His efforts were ably seconded by the doughty margrave, Gero, of the southern Wendish mark. By their strenuous exertions the Slavs were for the time driven away from German territory, and German rule was extended as far as the Oder, so that a whole ring of organised marchlands protected the northern and eastern frontiers. These marks became vigorous military states, possessing more energy and martial prowess than the purely Teutonic lands west of the Elbe, and destined on that account to play a part of extreme prominence in the future history of Germany. Owing their existence to the goodwill and protection of the king, and having at their command a large force of experienced warriors, the new margraves or counts of the marches, who ruled these regions, gradually became almost as powerful as the old dukes, and, for the time at least, their influence was thrown on the side of the king and kingdom. Under their guidance, the Slav peasantry were gradually Christianised, Germanised, and civilised, though it took many centuries to complete the process. Even to this day the place-names in marks like Brandenburg and Meissen show their Slavonic origin, and a Wendish-speaking district still remains in the midst of the wholly Germanised mark of Lausitz. To these regions Otto applied King Henry’s former methods on a larger scale. Walled towns became centres of trade, and refuges in times of invasion. Monasteries arose, such as Quedlinburg, and that of St. Maurice, Otto’s favourite saint, at Magdeburg. A whole series of new bishoprics—Brandenburg and Havelberg, in the Wendish mark; Aarhus, Ripen, and Schleswig, in the Danish mark—became the starting-points of the great missionary enterprise that in time won over the whole frontier districts to Christianity. Hamburg became the centre of the first missions to Scandinavia. Never since the days of Charles the Great had the north seen so great an extension of religion and culture. There was many a reaction towards heathenism and barbarism before the twelfth century finally witnessed the completion of this side of Otto’s work.