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:The Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.