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 Jan Hus Memorial stands at one end of Old Town Square. The huge monument depicts victorious Hussite warriors and Protestants who were forced into exile 200 years after Hus, and a young mother who symbolises national rebirth. The monument was so large that the sculptor designed and built his own villa and studio where the work could be carried out. It was unveiled in 1915 to commemorate the 500th anniversary of Jan Hus' martyrdom. The memorial was designed by Ladislav Šaloun and paid for solely by public donations.
Born in 1369, Hus became an influential religious thinker, philosopher, and reformer in Prague. He was a key predecessor to the Protestant movement of the sixteenth century. In his works he criticized religious moral decay of the Catholic Church. Accordingly, the Czech patriot Hus believed that mass should be given in the vernacular, or local language, rather than in Latin. He was inspired by the teachings of John Wycliffe.