Alt-Schauenburg is a ruined castle in the commune of Frenkendorf. It is located near the border of France and Germany, and little of the castle remains because of geological events.
There were two Schauenburg castles near Frenkendorf, Alt-Schauenburg about 1 km southwest of the village on the top of Chleiflüeli hill and Neu-Schauenburg to the west. Alt-Schauenburg was probably built around 1275 as the seat of a junior branch of the Schauenburg family. The castle was occupied for less than a century. The 1356 Basel earthquake destroyed much of the castle and shortly thereafter the ruins were abandoned. The ruins were gradually buried, until 1949-50 when they were excavated and repaired. In 1976-77 additional construction helped preserve the site.
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.