Page "Danevirke" Paragraph 4

According to written sources, work on the Danevirke was started by the Danish King Gudfred in 808.

Fearing an invasion by the Franks, who had conquered heathen Frisia over the previous 100 years and Old Saxony in 772 to 804, Godfred began work on an enormous structure to defend his realm, separating the Jutland peninsula from the northern extent of the Frankish empire.