The history of Welzheim reaches back to the time of the Roman Empire. Around the year 160 AD the Romans founded a colony and two forts there; linguists reconstructed their Latin name as (Castra) Valentia on the base of the town's medieval name Wallenzin first mentioned in 1181. In 1980, for the 800th anniversary of the founding of the city, the eastern part (Ostkastell) of the ruins was reconstructed, and in 1993 an archeological park was founded on the site. It contains replicas of Roman stone monuments and explanatory plaques about Roman Welzheim. The western part of the fort was built over through the years.
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.