Kuusisto Castle was a medieval episcopal castle built in the beginning of the 14th century. It was the main residence of Finnish Catholic bishops until 1522, when the last bishop Arvid Kurki drowned when fleeing from Danish soldiers.
Kuusisto heyday was in the in the beginning of the 15th century, when bishop Maunu Tawast spent lot of time and money to enlarge the castle. The castle was ordered to be demolished during the Protestant Reformation in 1528 by the king Gustav I of Sweden. Stones of the castle were later used in renovations of Turku and Kastelholma castles.
Excavation and reconstruction work on the remaining ruins began in 1891.
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.