The Mandraki was the military harbour and was guarded by a tower built between 1464 and 1467 by the Grand Master Zacosta at the end of the natural mole. After the siege of Rhodes in 1480 the Grand Master d'Aubusson added a bastion around the tower transforming it into a guard fortress on the sea.
The castle was built at the place of an old Byzantine church dedicated to St. Nicholas, hence the name. Since the 17th century, a lighthouse was added at the top.
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.