Köngen is in the area of the Roman settlement Grinario that was built around 100 AD around a Roman castra. Grinario was the endpoint of the Roman road 'Neckar-Alb-Aare' which originated in Windisch (Switzerland). In 260 the place was destroyed by the Alemanni and the Romans were expelled.
Johann Eberhard Roser discovered some traces in 1782 and started excavations here one year later. At present it is the site of an archaeological park and museum. The earliest reconstructions were those of a Roman corner tower, the originals of which were discovered in 1855. The tower was ready in 1911 and the 'Köngener Römerkastell' with its great viewpoint and small indoor museum soon became a popular excursion.
In World War II and the period short after, the tower was damaged and it was not open to public until 1949. In the 1970s, the tower was fully renovated and a small museum was built next to it.
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.