Grønsalen or Grønjægers Høj is about 100 metres long and 10 metres wide, which makes it Denmark's largest long barrow and is widely recognised as one of Europe's outstanding ancient monuments. The barrow, rising over a metre above the surrounding area, is encircled by 134 large stones. The grave, at the centre, is covered with earth and contains three burial chambers, two of which are open. It is not known when they were first opened or what was found inside. The long barrow was examined in 1810 by Bishop Münter and was protected by law after that. On the basis of its shape, the barrow has been dated as Neolithic, approximately 3500 BC. The first historical reference to the site was in ca. 1186 when it was called Grónesund.
References:Sigmaringen Castle was first mentioned in the year 1077 in the chronicles of Petershausen monastery. The oldest parts of the castle are concealed beneath the alterations made during the 17th and the 19th centuries. The secret of the earliest settlement built on this defendable rock will never be fully revealed: large-scale excavation work would be necessary, which the extensive land development renders impossible. Judging from the many Roman remains unearthed in the area around Sigmaringen, the 12th century keep known as the 'Roman Tower' could be traced back to a Roman predecessor.
The castle remains that have been preserved (gate, great hall and keep) date back to the Staufer period around 1200. The castle remains were integrated into subsequent buildings. The foundations of the castle buildings are to a large extent identical to the surrounding castle wall.
These remains give us a good idea of how the castle might have looked during the 12th century.