Neu-Thierstein Castle was founded in 1100 and a new building was built around 1294/95. The castle was apparently built by the Saugern-Pfeffingen family as a seat for a Kastvogtei (or a vogt with authority over a religious structure) who ruled over Beinwil Abbey. The first mention of the castle comes from 1321 when it was called Bello. As heirs of the Saugern-Pfeffingen family, in the late 12th Century, the count of Thierstein took over the castle and the Kastvogtei post. By 1400 it was known as Thierstein Castle. The castle was occupied by the Canton of Solothurn in 1445, 1467 and 1499. After the extinction of the counts of Thierstein in 1522, Solothurn acquired the castle and made it the administrative center of the secular bailiwick of Thierstein. In 1798 the castle was sold for demolition. However, in the 19th Century, the destruction of the castle are stopped by private individuals who wanted to repair the ruins. The ruins were surveyed and archeologically investigated in 1985.
The castle ruins are on a small rocky outcrop above the municipality of Büsserach. The ruin consists of a residential tower with ancillary buildings. Traces of a bailey and the old gate house on the south side can still be seen.
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.