Grüningen castle was built before 1229. It was in early times owned by the Counts of Regensberg. From the original castle only the Palas exists. At the place of the today's church stood a chapel since at least 1396, which was extended 1610. In 1782 it was demolished and rebuilt in its early Classicist style.
For centuries, the castle has been the residence of bailiffs. They have been assigned for the administration and justice over the large area at the Zurich Oberland from Lake Zurich to the Töss Valley. The museum gives an overview of the history of Grüningen, the bailiwick and sovereignty with its castle and country town. The museum is located at the first floor of the castle building and is open from April to October.
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.