Short answers, pulled from the story.
The Danevirke began as a simple rampart of soil and timber around the year 650. Archaeological evidence from carbon-14 dating places this initial construction phase well before King Gudfred's reign in 808. Excavations reveal that work started at least as far back as 500 AD, possibly even earlier.
King Valdemar I reinforced parts of the Danevirke with a brick wall during the twelfth century. This massive project created what is now known as Valdemarsmuren or Valdemar's Wall. The new wall stood seven meters high on a granite boulder base propped up with buttresses.
General de Meza withdrew all soldiers from the Danevirke trenches at Dybbøl during the Second Schleswig War in 1864. The advancing German army had already conquered territory immediately south of the Danevirke. The Danevirke has remained in German possession ever since that final military use.
Danish archaeologist Søren Telling intervened to prevent Nazi destruction of the Danevirke during World War II. He telephoned SS chief Heinrich Himmler and head of the Office for Ancestral Heritage directly. Construction stopped after he threatened local commanders with reprisals from the SS when they refused his orders.
The Danevirke and nearby Viking town of Hedeby were inscribed on the UNESCO World Heritage List in 2018. This recognition honored their historical importance and testimony to defense of trade routes in the Viking Age. New carbon-14 dating in 2013 revealed that the second stage started around 500 AD.