The Tregiffian Burial Chamber is a Neolithic or early Bronze Age chambered tomb. It is a rare form of a passage grave, known as an Entrance grave. It has an entrance passage, lined with stone slabs, which leads into a central chamber. This type of tomb is also found in the neighbouring Isles of Scilly.
The large stone grave, half of which was covered by a road in 1846, was, unlike Cornish quoits, for the most part covered with soil, with only the entrance exposed. From the edge of the site a passage, covered by four 3 m long stones, led to the 4-metre deep grave chamber. In front of the chamber, a cross-lying ornate stone, with cup-and-ring markings, formed a barrier. The original stone is in Truro, in the Royal Cornwall Museum, the local stone is a replica. Inside the tomb there was the chamber grave, which consisted of upright stones and a cover slab. Tregiffian probably formed a holy place with the Merry Maidens and other sites.
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.