Danewirk, ancient frontier earthwork of ramparts and ditches built by the Danes across the neck of Jutland in order to block Frankish expansion into the area. It ultimately extended to an overall length of about 19 miles (30 km) from just south of the town of Schleswig to the marshes of the river Trene near the village of Hollingstedt. The structure was built between about 650 and 1200 ce. The earliest stages of the fortification have been carbon-14 dated to the second half of the 7th century. The Danewirk was reconstructed and enlarged about 808 by Godfrey (Gudfred), a king of Denmark. In 934 it was penetrated by the German king Henry I, after which it was extended by the Danish king Harald I Bluetooth (c. 940–c. 985), but it was stormed by the German emperor Otto II in 974. After the union of Schleswig and Holstein under the Danish crown in the 15th century, the Danewirk fell into decay. In 1848, though, it was hastily strengthened by the Danes, who were, however, unable to hold it in the face of the superiority of Prussian artillery, and on April 23 it was stormed.
From 1850 onward, the Danewirk was again repaired and strengthened at great cost and was considered impregnable. In the German-Danish War (1864), nevertheless, the Prussians turned it by crossing the Schlei inlet to the northeast, and the Danes abandoned it on February 6 without a blow. The Prussians thereupon destroyed the Danewirk, but a long line of imposing ruins still remains. The systematic excavation of them, begun in 1900, yielded some notable finds, especially of valuable runic inscriptions. In 2012 archaeologists found ruins of a garrison town near the Danewirk, including evidence of some 200 buildings—notably a Viking longhouse that appeared to have been attacked and destroyed by fire. The excavators theorized that the site may be that of Sliasthorp, the very base used by Godfrey and his men during his reconstruction of the Danewirk.