Kaštilac was a fortress of a Benedictine Monastery. Kaštilac is the only castle of seven castles in Kaštela. King Zvonimir of Croatia of the Svetoslavić branch of the House of Trpimirović, had donated to a Benedictine from Split possession in the Kaštela field (Kaštelansko polje) in 1078. In the 12th century Benedictine built a church of St. Cosmas and Damian in the Romanesque Architecture style.
Due to the Turkish threat, Benedictine nuns completed the construction of a fortified settlement Kaštilac on the islet of Gomilica in 1545. Entrance to the castle is protected by high tower over the stone bridge with arches. Kaštilac is 40 metres away from the shore, connected with the stone bridge, which was made only of stone. Fortress has square floor plan and in the past, at the entrance was located drawbridge. After the fortress was completed, within the fortification walls arrived inhabitants of the Upper and Lower Kozica, after the Turks had devastated their former villages. In the 17th century the tower lost its defensive function.
References: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.