General Notes

King in Jutland
He was also King of Rustringen and Jutland, Regent in the lands of the Stormarn and Obotrites.

Harold's title of Ruler of Jutland seems a bit pretentious; he was granted the fife of Rustringen by the mouth of the Weser in 826. First, he had been baptized with the Emperor as godfather, from whom he received munificent christening presents.
He is credited by some as having built the Danevirke, one of the largest of northern Europe's ancient defensive works. The entire complex consists of several different ramparts, about 18 miles in length, running from the Schlei fjord in the east to the rivers Rheide and Treene in the West. The first phase, the North Wall, was built in 737 (dated by dendrochronology); the East Wall, protecting the Svansen peninsula from the south is believed to be of about the same age (no scientific dating yet). The 'Great Wall', the third phase, makes a zigzag across the country from Hedeby in the east to nearly Hollingstedt in the west where the two rivers meet to form the Eider. This third section has been dated to 951-968, suggesting that Harald 'Bluetooth' (who was responsible for vast structures elsewhere in the country) commissioned it.
In 974, the year after Emperor Otto I died, the Danevirke was taken by Otto II.
The Danevirke was expanded during the 11th and 12th centuries to defend against both Slav and German attacks. During the second half of the 12th century, Valdemar 'The Great' added a wall of brick. It was extended for the last time in 1864 when the Prussians attacked Denmark.