Koroinen was the residence of Bishop of Finland between years 1229 and 1300. in 1300 the bishop seat was moved couple of kilometres further down the River Aura, to the present-day Cathedral of Turku. There were at least two wooden churches in Koroinen built in the 14th century. The latest one was probably a stone church. It was later destroyed by the Victual Brothers in 1396.
Archaeologists have found remains of three wall grounds, which were probably part of the church. Today there are a white, wooden memorial cross and some stone foundations still remaining on site. The landscape of the site is anyway unique in Finland. You can see three medieval church from there: The Turku Cathedral, St Mary's Church and St. Catherine's Church.
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.