The ruins of Frohberg castle is on a rocky ridge at the upper end of the Klus valley on the way to the old Platten pass road. The stronghold at Frohberg was first mentioned in 1292 when Conrad I Schaler 'de Vroberg' was mentioned. It is likely that the castle was built by the Schaler family in the second half of the 13th Century. Although the castle dominated the way across the Platten pass (between Birseck and Laufental), the location of other castles in the immediate vicinity, suggests that the motivation for the castle was not the collection of tolls, but power games between the families of the Schaler and Münch.
The castle was perhaps never quite finished or it castle was damaged during the Basel earthquake of 1356 and was not repaired. In any case, in the 14th Century, the remains were given as a bishop's fief to the counts of Thierstein-Pfeffingen. This fief was not so much about the ruins, which needed costly repairs, but the assets related to the castle including tax collection and court rights.
The ruins are widely scattered and consists of an extended main castle, surrounded by different approach obstacles. So far, the ruins have not yet been investigated archaeologically and so only rough interpretations are possible. The main castle was formed by a housing tract and a curtain wall. The wall follows the irregularly extending ledge. The massive living area consists of two parts and a smaller west building with irregular floor plan as a residential tower. The thick walls were up to 3 meters thick and built from rough cut stone. To the east, adjoining the living area, is an elongated building that served as an administrative and residential building. On the northwest and northeast sides the remains of outlying estates are visible.References:
Dating from the 15th century, Kisimul is the only significant surviving medieval castle in the Outer Hebrides. It was the residence of the chief of the Macneils of Barra, who claimed descent from the legendary Niall of the Nine Hostages. Tradition tells of the Macneils settling in Barra in the 11th century, but it was only in 1427 that Gilleonan Macneil comes on record as the first lord. He probably built the castle that dominates the rocky islet, and in its shadow a crew house for his personal galley and crew. The sea coursed through Macneil veins, and a descendant, Ruari ‘the Turbulent’, was arrested for piracy of an English ship during King James VI’s reign in the later 16th century.
Heavy debts eventually forced the Macneil chiefs to sell Barra in 1838. However, a descendant, Robert Lister Macneil, the 45th Chief, repurchased the estate in 1937, and set about restoring his ancestral seat. It passed into Historic Scotland’s care in 2000.
The castle dates essentially from the 15th century. It takes the form of a three-storey tower house. This formed the residence of the clan chief. An associated curtain wall fringed the small rock on which the castle stood, and enclosed a small courtyard in which there are ancillary buildings. These comprised a feasting hall, a chapel, a tanist’s house and a watchman’s house. Most were restored in the 20th century, the tanist’s house serving as the family home of the Macneils. A well near the postern gate is fed with fresh water from an underground seam. Outside the curtain wall, beside the original landing-place, are the foundations of the crew house, where the sailors manning their chief’s galley had their quarters.