During the 12th century and onwards, Osthoffen became a fortified castle. Due to a siege lead by the Bishop of Strasbourg in 1410, fire raged through the building, depriving it of its 3rd floor. It was rebuilt as a Rennaissance estate and a private dwelling. More changes were made in the 18th century: courtyard walls were removed and new wide windows enlightened both the façade and the interior. The castle lost its towers during the French Revolution by order of the new authorities, one of the first works undertaken by the new owner was to rebuilt them. Today Château d'Osthoffen is a hotel and event venue.
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.