Constantina Castle was dates back to the Moorish period, when it defended the north-south road towards Seville. It was conquered by Christians in 1274 and by the privilege of King Alfonso X in 1253, it passed to the Alfoz of Seville. In 1810, it was used as a fortified point during the period of the French invasion of the town.
Today the castle of Constantina consists of a polygonal enclosure of about ninety metres in diameter, which has an external defence in the form of a barbican, of which there are sufficient remains to affirm that it completely surrounded the enclosure. Most of this area has been preserved, and two of the seven towers that flanked it are still complete, containing magnificently built circular vaulted chambers.
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.