The Danevirke, The Work of the Danes, was a stone wall that separated medieval Denmark from the German empire to the south. About 19 miles long, it was first built in the 7th or 8th century and regularly rebuilt until the end of the Middle Ages. Archaeologists recently found the remains of a great stone gateway that was, our sources tell us, the only one through the wall. Nearby was an inn for travelers. Article here, with lots more pictures.
It goes rather against our image of the Vikings to think of them building a 19-mile-long stone wall, but the kings of Denmark were quite powerful from an early period. Individual Viking lords could mount raiding expeditions with one or two ships, and they sometimes worked together to launch larger raids, but those huge Viking invasions of England and Normandy were organized and led by kings.