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. A first castle was probably built for the Lords of Schauenburg in the 11th century, however nothing is known about it. In the 13th century the original castle was replaced by a new castle. The 1356 Basel earthquake destroyed much of it, but unlike Alt-Schauenburg, it was quickly rebuilt. The Schauenburg family became extinct in 1385 and the castle became a fief under the Bishop of Basel. In 1397 it was sold and then over the following centuries passed through several additional owners. However, by the 15th century, it had been abandoned and began to fall into ruin.
By 1480 a group of Beguines lay sisters had occupied the towers of Neu-Schauenburg and established a cloistered house in the ruins. The order remained at Schauenburg until 1523. In 1691 mineral springs were discovered and a bath house was built near the castle ruins. In 1792 a small country estate with attached fields was built near the ruins.
Neu-Schauenburg was built before Alt-Schauenburg, but because Alt-Schauenburg was destroyed and abandoned first, it came to be known as the old castle.
The castle was built on a rocky spur above the valley. It has an irregular floorplan and a gate on the north side with a 2m thick castle wall that follows the edge of the spur. The top of the spur is divided down the center by a narrow band of rock into higher eastern and lower western halves.
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.