The Danevirke
A system of ramparts, walls, and moats running across the base of the Jutland peninsula at Schleswig, built from the 5th century onward to guard the southern border of the Danish realm. It was manned, extended, and rebuilt at many points in its history, most notably under King Gudfred in 808, and it is described in early royal chronicles. The surviving stretches, some still under the modern border, are a UNESCO World Heritage site. The Danevirke remained a working fortification into the 19th century, making it one of the longest-used defensive lines in northern Europe.