The history of Venice begins when people from the mainland came to the marshy islands for refuge. According to tradition, the citizens of Venice originated from nearby cities like Aquileia, who were fleeing successive waves of Germanic and Hun invasions. Over the next few centuries, these people slowly transformed the islands into what they are today. Because of their defensive position, the Venetians were basically able to maintain their independence from the invading barbarians, and thus essentially remain Roman. 
The collapse of the Western Roman Empire made them naturally lean towards remaining Eastern Empire. The Byzantine Emperor Justinian firmly brought Venice under the rule of the Byzantine Empire. At first the Patriarchs of Aquileia claimed authority over the Venetian lagoon, but the invasions devastated local episcopal sees, leading to creating an ecclesiastical vacuum. Aquileia eventually had two competing bishops, weakening the position, while the Bishopic of Altino moved to Torcello in 647. Later an episcopal see was established on the island of Olivolo (later called Castello) in 775.
Over time, as the Venetians grew in power, their trade to the east also grew. At first, though, Torcello, located north of Venice, was the center of power in the lagoon, but later was eclipsed by Venice. Venice became important as Crusaders began to increase connections with the east.

In 1082, important political agreement between Venice and the Byzantine Empire increased Venice’s economic presence and status throughout the Byzantine Empire.  In the preceding years, the Venetian fleet came to the aid of Constantinople, then under attack by the Normans.  For their service in the defeat of the Normans, the Emperor greatly awarded them with a variety of different items, including annual financial grants and grand titles given to Venetian citizens, but the most important clause involved the Venetian trade status throughout the Byzantine Empire. In 1171, however, the relationship between Venice and the Byzantine Empire changed.  The Byzantine Emperor falsely accused the Venetians of an attack on a Byzantine province and imprisoned nearly all the Venetian citizens in Constantinople as punishment and revenge. This would indirectly lead to the Venetians leading the Fourth Crusade to sack Constantinople in 1204. Even after the Byzantines regained control of Constantinople, Venetians were economically powerful and would continue to have strong connections to Constantinople even in the Ottoman Era. 

The long connections between Constantinople and Venice can clearly be seen the unique Venetian-Byzantine architecture found across the city. The best example of this is the Basilica of Saint Mark (San Marco). It follows the model of the Church of the Holy Apostles, where Constantine and other Byzantine emperors were once buried. It also has the magnificent golden mosaics, typical of Byzantine church art. San Marco also has a spectacular alterpiece, Pala d’Oro, which includes Byzantine enamels. Venetians also brought back loot from Constantinople, preserving some of the best examples of Byzantine artifacts. This most spectacular example is the Triumphal Quadriga, four horses which were likely located at the Hippodrome. In addition, there is a marble sculpture of the Tetrarchs, possible including Diocletian or Constantine among the figures. Both of these works actually date before Constantine moved the capital to Constantinople, thus are examples of spolia. The so-called Pilastri Acritani (Pillars of Acre) are located nearby, which came from the Church of St. Polyeuctus, once the largest church in Constantinople. San Marco also has a large number of Byzantine pieces in its treasury. It also contains works of Islamic and even Antiquity origin. Yet these are not the only pieces coming from Constantinople or the Eastern Mediterranean. The Piraeus Lion, on display at the Arsenal, is an ancient statue, once located in Piraeus, the harbor of Athens. The Column of the Lion and the Column of St. Theodore of Amasea, now located in Piazzetta San Marco, are more examples of works coming from the East. There are other examples of Venetian-Byzantine architecture, such as the Murano Church and the Ca' da Mosto. While radically different in outlook and history, the richest of Byzantium can perhaps best be seen in the great city of Venice.