Hainburg Castle, also known as Heimenburg, was built by Henry III the Holy Roman Emperor around 1050 to protect the traffic in Danube. It was enlarged in the mid-13th century. The castle was besieged in 1619-1620 by Hungarian army and conquered in 1683 by Ottomans.
After the new residence was built in 1742 the old castle was left to decay. Today Hainburg castle can freely be visited. A nice castle ruin, giving great views over the medieval town below and the surrounding countryside.
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.