In the midst of the Norse invasions, Hertfordshire was on the front lines of much of the fighting. King Edward the Elder, in his reconquest of Norse-held lands in what was to become England, established a "burh" or fort in Hertford, which was to curb Norse activities in the area. His father, King Alfred the Great, established the River Lea as a boundary between his kingdom and that of the Norse lord Guthrum, with the north and eastern parts of the county being within the Danelaw. There is little evidence however of Norse placenames within this region, and many of the Anglo-Saxon features remained intact to this day. The county however suffered from renewed Norse raids in the late 10th to early 11th centuries, as armies led by Danish kings Swein Forkbeard and Cnut the Great harried the country as part of their attempts to undermine and overthrow English king Athelred the Unready.