A castle in Arbon is first mentioned in 720 in a history of St. Gall Abbey. It stood on or near the site of the Roman era Arbor Felix fortress from 250 AD. After the Romans retreated south of the Alps around 400, the old fortress was abandoned. Sometime later a Frankish castle was built in Arbon probably for the Frankish royal family. By around 700, Arbon and presumably the castle, were the property of the diocese of Constance and an ecclesiastic overseer or bailiff ruled over Arbon.
The oldest part of the current castle is a 13th-century, originally free standing, residential tower. The tower may have been built on the foundation of an earlier building. The lower walls are up to 3.2 meters thick. Inside the castle there are two Romanesque fireplaces. In 1515-20 the Bishop Hugo von Hohenlandenberg rebuilt the castle to its current appearance. The old tower was rebuilt into a U shape. The upper most story, the gables and the hip roof are also from this reconstruction. The outer wall of the castle moved to stand exactly over the north-west corner of the Roman era fortress.
In 1822 the Stoffel silk ribbon weaving company moved into the castle. They remained there until 1907. In 1911 the castle was bought by Adolph Saurer, the founder of Adolph Saurer AG. He built workshops around the castle building as he experimented with motors, trucks and machinery. In 1944 his son, Hippolyt Saurer sold the castle to the city. The city renovated and expanded the castle and in 1967 opened the Historisches Museum Arbon (Arbon History Museum).
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.