King Doniert's Stone consists of two pieces of a decorated 9th-century cross near St Cleer. The inscription is believed to commemorate Dungarth, King of Cornwall, who died around 875.
The site consists of the remains of two granite cross-shaft fragments dating from the 9th–11th century, and an underground passage and cross-shaped chamber below the crosses, thought to be either the remains of tin workings or a possible oratory. The northern cross, the Doniert Stone, is 1.37 metres high with panels of interlace decoration on three sides and inscription doniert rogavit pro anima carved in half uncial or insular script. The inscription translates as 'Doniert has asked [for this to be made] for his soul['s sake]'.
The inscription is thought to refer to the local ruler Dumgarth (or Dwingarth), who is recorded in the early Welsh chronicle known as the Annales Cambriae as having drowned in around 875 AD. It has a mortise slot and a plinth at the base. It is notable for being the only inscription to a Cornish King also known from documentary sources.
The southern cross, sometimes referred to as the Other Half Stone, is 2.1 metres (6 ft 11 in) high with a panel of interlace decoration on the east face, a broken mortise slot at the top and a plinth at the bottom.
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.