During the Early Middle Ages there was a small fort and church on the top of the Thun castle hill. The castle was built between 1180 and 1190 by Duke Berthold V of Zähringen, who constructed the still preserved keep to the level of the Knights' Hall (Rittersaal). The 14 m tall Knights' Hall was built as the centerpiece of a monument to Zähringen power. However, the family never lived in the castle, preferring Burgdorf Castle.
In 1218 it was inherited by the House of Kyburg, who built the upper levels above the Zähringen castle. A quarrel over who would rule the southern Kyburg lands led, in 1322, to Eberhard II von Kyburg murdering his brother Hartmann II at the castle. To protect his newly acquired land from the Habsburgs Eberhard II then sold them to Bern and was promptly given them back as a fief. The Kyburgs ruled over the region for nearly two centuries until a failed raid by Rudolf II on Solothurn, in 1382, started the Burgdorferkrieg. After several decisive Bernese victories the Kyburgs were forced to concede an unfavorable peace. In 1384 Bern bought Thun and Burgdorf, the most important cities of the Kyburg lands. The castle came under Bernese control and became the seat of their local administration.
The massive roof (1430–36) comes from the Bernese period. Due to the lack of residences in the castle, in 1429, an administrative and residential wing was added to the west of the keep, built in late Gothic style, and known as the 'new castle'. The castle was the seat of the local court and since at least the 17th century there was a prison under the roof of the donjon. In 1886 a new prison was built on the castle grounds. Two years later, in 1888, the museum opened in the castle. For a time the jailer was also the ticket seller and guard for the museum.
The castle museum is housed in the five floors of the tower, and includes cultural and historic displays showing the development of the region over some 4,000 years. It is open daily between February and October, and on Sundays only for the rest of the year. The great hall is used for concerts or plays, and can be hired for private events.
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.