While the builder of the Aarberg Castle is unknown, the city itself was founded between 1220 and 1225 by count Ulrich III of Neuchâtel. The count had recently acquired the rulership over this region and needed a central location from which to rule. The island and the key bridge was a natural location for a town. The castle may have been built around the time of the founding of the city.
The city was besieged in 1339, 1382 and 1386 but not taken. In 1358 the Graf (or Count) Peter von Aarberg was in financial difficulties and began looking for someone to buy the city. After years of unsuccessful attempts, in 1377-79 he was able to sell the city and his rights as ruler to Bern. The Bernese bailiff took up residence in Aarberg Castle in 1379. In 1380, the old castle roof was replaced with a new shingle roof. In 1419 and again in 1477, the town and castle of Aarberg were nearly destroyed in a fire. Both the town and castle were rebuilt following the destructive fires.
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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.