Ragnhildsholmen was a medieval castle built by Håkon Håkonsson (Haakon IV) of Norway around the year 1250. The castle was first time mentioned in 1275. In 1304 it was donated to duke Erik in 1304 and saw the power struggle between Swedish Kings Birger and Magnus III. After the near Bohus Fortress completed, Ragnhildsholmen lost its purpose. It was demolished and stones were probably used to build Bohus. Today impressive ruins still stands on the site.
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.