The oldest parts of the defensive works…
676 CE to 819 CE
The size and number of troops needed to man it indicates a quite powerful ruler in the area, which might be consistent with the kings of the Frankish sources.
In 815, Emperor Louis the Pious attacks Jutland apparently in support of a contender to the throne, perhaps Harald Klak, but is turned back by the sons of Gudfred, who most likely are the sons of the above-mentioned Gudfred.
At the same time St. Ansgar travels to Hedeby and starts the Catholic Christianization of Scandinavia.