Aquileia was the most prosperous city of the empire after Rome, having 600,000 inhabitants in the days of its prosperity. The fleet which kept the capital in communication with the eastern coast of the Adriatic, and so with Liburnia, Giapidia, Pannonia, and the Levant, had a station there. Trajan took the division which was called Aquileian or Venetian from the Pretorian fleet at Ravenna. It had charge of the Upper Adriatic from Ancona to Zara, and of the shore from the Adige to the Arsa. After the Greeks lost Ravenna to the Lombards the station of the fleet was moved to Zara. Shortly before, in 743, the exarchate included the Dalmatian islands, and also the cities of Zara, Traù, Spalato, and Ragusa. The Slavs occupied Dalmatia in 640-642. Paulus Diaconus says that they crossed to Siponto in 649 and sacked several places near. The annals of Bari (926) speak of the siege and capture of Siponto by a Slav king, Michael, possibly the husband of Queen Helena, who is named on his wife's sarcophagus found on the island in the Jader, near Salona, as described in the chapter on Spalato. In the ninth century the Narentans helped in driving the Saracens from Monte Gargano.
The bishop of Torcello had possessions in Cittanova and Muggia, which were confirmed to him in 1177 by Frederick Barbarossa. The see of Grado had rights and possessions on the islands, and in Istria, at Trieste, Capodistria, Pirano, Cittanova, Parenzo, Pola, and Castel S. Giorgio, but the actual power was in the hands of the patriarch of Aquileia, who several times settled matters with his adversaries by giving them things which really belonged to Grado. With the increase of the Venetian power to the point at which the coast-towns were practically forced to yield themselves to her supremacy, Istria and Dalmatia became pawns in the political game which was played in Italy, and the reciprocal influences of the two shores became principally artistic and individual, rather than corporate or national.
Artists of both shores worked indiscriminately on either side of the Adriatic, as may be divined from the similarity of style in many of the buildings and in much of the decorative work, even without the documentary evidence which is often available. It is to be expected that between the early basilicas of Ravenna and of Pola there should be a great resemblance; but at Parenzo, also, there is a likeness to both those places, and it seems probable that the same school of artists worked upon the mosaics there and at S. Maria in Cosmedin, Ravenna. The decoration in _opus sectile_ also has resemblances, but these seem more probably due to direct Byzantine influence, since, both at S. Sophia, Constantinople, and S. Demetrius, Salonica, the same form of decoration occurs; and it is pretty well established that there was a regular export trade in carved capitals and columns from Constantinople, the same patterns occurring in many places far apart from each other. Comacine work is frequently met with all down the eastern coast as far as Cattaro, as in Lombardy and the Venetian territory. The building at Ravenna known as the Palace of Theodoric resembles the Porta Aurea, Spalato, in its decoration of columned niches; and the material of his mausoleum, Istrian stone, inclines one to look across the sea for the inspiration of the design (which may possibly be a Gothic imitation of the mausoleum of Diocletian), though it must be remembered that Theodoric sent an architect to Rome to study the ancient buildings.