With the Saracens in Spain, however, the Emperor had constant trouble. They made repeated incursions across the Ebro, into the Spanish Mark, and ravaged the shores of Majorca, Minorca and Corsica, which belonged to the Frank Empire. Moreover, the extension of his frontier on the east brought Charlemagne into collision with the Slavonic tribes in the territory now belonging to Prussia beyond the Elbe, Saxony and Bohemia. He easily defeated them, but could not check their plundering and roving propensities. In the year 808, Holstein as far as the Elbe was invaded by the Danish king, Gottfried, who, after returning home with much booty, commenced the construction of that line of defence along the Eider river, called the _Dannewerk_, which exists to this day. Charlemagne had before this conquered and annexed Friesland. His Empire thus included all France, Switzerland and Germany, stretching eastward along the Danube to Presburg, with Spain to the Ebro, and Italy to the Garigliano river, the later boundary between Rome and Naples. There were no wars serious enough to call him into the field during the latter years of his reign, and he devoted his time to the encouragement of learning and the arts. He established schools, fostered new branches of industry, and sought to build up the higher civilization which follows peace and order. He was very fond of the German language, and by his orders a complete collection was made of the songs and poetical legends of the people. Forsaking Paris, which had been the Frank capital for nearly three centuries, he removed his Court to Aix-la-Chapelle and Ingelheim, near the Rhine, founded the city of Frankfort on the Main, and converted, before he died, all that war-wasted region into a peaceful and populous country. [Sidenote: 810.] No ruler before Charlemagne, and none for at least four centuries after him, did so much to increase and perpetuate the learning of his time. During his meals, some one always read aloud to him out of old chronicles or theological works. He spoke Latin fluently, and had a good knowledge of Greek. In order to become a good writer, he carried his tablets about with him, and even slept with them under his pillow. The men whom he assembled at his Court were the most intelligent of that age. His chaplain and chief counsellor was Alcuin, an English monk, and a man of great learning. His secretary, Einhard (or Eginhard) wrote a history of the Emperor's life and times. Among his other friends were Paul Diaconus, a learned Lombard, and the chronicler, Bishop Turpin. These men formed, with Charlemagne, a literary society, which held regular meetings to discuss matters of science, politics and literature.