Heidenheim was first mentioned in the year 742. During that time the double monastery of Heidenheim am Hahnenkamm (housing monks and nuns), was founded by Saint Willibald and was later led by Saint Walpurga who became abbess after his death.
The first stone church was inaugurated in 778 and the next one hundred years later. The remains of current church date from the Romanesque style build constructed in 1182-1188.
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.