Another of the Trelleborg-style Viking circular fortresses has turned up at Vallø on Zealand:
http://www.telegraph.co.uk/news/worldnews/europe/denmark/11079056/Viking-ring-fortress-discovered-in-Denmark.html (http://www.telegraph.co.uk/news/worldnews/europe/denmark/11079056/Viking-ring-fortress-discovered-in-Denmark.html)
Similar article on this site here: http://gizmodo.com/archaeologists-just-discovered-a-1-000-year-old-viking-1632006728
This one has some pictures of what it might have looked like and how they located it.