Between 1115 and 1118 the Augustinian monastery was established in Sankt Märgen by Count Bruno von Haigerloch-Wiesneck, Chancellor of Henry V and Provost of Straßburg, as competition to the Zähringen-founded Benedictine monastery of St. Peter auf dem Schwarzwald. The town and the monastery were dependent upon each other for many years. In 1716 a two-steepled Baroque monastery church was built, and the life of the community underwent a revival, until 1806 when the monastery was given up.
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.