St. Peter's Church have been probably built in two phases during the 12th century.The eastern part with chancel, transept and central tower were erected first during the late 1100's, while the nave and the present west tower were added later.
According the tradition the church was used as a bishop’s cathedral until 1130 when the bishop's seat was moved to Gamla Uppsala. The another legend believes the church has been a royal church. After a fire in the 1600s, the church has stood as a ruin.
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.