Britons held out in their fortress at Dumbarton until 870, when, after a siege of four months, the place was taken by Olaf and Imar, the joint-reigning Norse kings of Dublin. These kings, with a fleet of 200 ships, returned next year to Dublin, "bringing a great spoil of men, Angles and Britons and Picts, in captivity." The Northumbrian kingdom, even south of the Tweed, was crumbling away. In 867, the Norsemen occupied York and defeated the Angles who came against them; and in 876, Halfdene, a Norse commander, parcelled out the remnant of Northumbria among his followers, who settled upon the lands, says the Anglo-Saxon Chronicle, and thenceforth set about ploughing and tilling them. In the same year, 876, Rolf the Ganger, of the line of the Norse earls of the Orkneys, took possession of Normandy. Here it is well to consider the various fortunes of the Norsemen in different countries. About this period, they became masters of a large part of Russia. In France, they were able to wrest the northern seaboard, between Flanders and Brittany, from the powerful Frankish kings. Over England they effected a gradual conquest, which was only checked, not overcome by the stout resistance of Athelstan and Alfred. In 1013, the year before the battle of Clontarf, all England submitted to Sveinn, king of Denmark. The Normans mastered southern Italy and Sicily. But the Celtic countries, Ireland, Scotland, Wales and Brittany, though particularly exposed to conquest by a people who were then undisputed rulers of the seas on every side, yielded them only a small fraction of their mainland territories. The resistance of Scotland is especially noteworthy. From Norway and Denmark, Scotland was then two days' sail. All the islands and forelands of Scotland were occupied by the Norsemen--the Orkney and Shetland Islands, the Hebrides, Arran and Bute, Caithness, and the peninsulas of Argyle and Galloway; as well as the Isle of Man. But the recently established Scottish monarchy checked all further attempts at conquest, and ultimately recovered the whole country, both mainland and islands.