Before we start cataloguing the mysteries of Venice, we need to look at the powers which surround the city, and the history that embodies.
Venice was built by refugees, who took to the islands of their lagoon to avoid the waves of barbarians who entered Italy as the Empire in the West declined. They were not, however, conquered by these wandering people. It’s important to Venice’s identity that it is Rome Unfallen. For a long time Venice was, in some sense, distantly beholden to the Roman Emperor in Constantinople.
It is the only part of Italy that is explicitly not in the Carolingian Empire. Charlemange’s son Pepin attempted to invade Venice, on his father’s orders. He was able to take a sliver of their land, but he was unable to force their surrender. This is thought to have forced the Venetians deeper into the lagoon, around the Rialto area, which is now the centre of their government. When he withdrew, Pepin, and his father, agreed that Venice belonged to the Empire of the East.
It’s in the Roman Tribunal because, I presume, whoever laid out the Tribunal maps just put all of modern Italy together, much as the Langedoc Tribunal is in the wrong place, and the Normandy Tribunal doesn’t include Norman possessions in either Aquitaine or England.
Although Venice is beholden to the Emperors, they are far away, and for many centuries now, have not really had the capacity to punish Venetian insubordination. The Venetians have the last republic in the West, excepting the Order. Power is effectively held by an elaborately constrained oligarchy of wealthy families. The Doge, the leader of Venice, is deliberately checked at virtually every turn, and the idea that the role might be inherited is abominable. Technically he’s a duke of the Eastern Empire, which grants him a certain amount of magic resistance.
Spiritual Powers
The Venetian Church is not a prominent landholder, so its potentates have a lot less money than elsewhere. Lacking money, they also lack military power. Venice sits out the Investiture Controversy, which pits the Pope against the western Emperor. It supplies aid to the Lombard League and Pope surreptitiously,, but acts as the broker between the two sides. The battles end in the Treaty of Venice. in 1177, where Venice’s allies basically get what they want.
Venice’s resident bishop is at Castello. It is part of the Patriarchate (essentially an archibishopric) of Grado. The Patriarchs of Grado have done business from Venice itself for decades, from a church which would otherwise be in Castello, and this causes tension. Grado is also in feud with the patriarchate of Aquileia, from which is was formed by a, now healed, schism centuries ago, and the Patriarchate of Zara (Zadar). Technically Grado is senior to Zara, but it is weird for an archbishop to be inferior to another archbishop – unique in Italy – and the Croato-Hungarians keep trying to be free of it.
Temporal Powers
More directly relevant to the history of the Venetians is city of Ravenna. It was the headquarters of the final Western Roman Emperors, and also the Byzantine governor of Italy, during the periodic re-establishments of Imperial rule. There are several occasions when the ruler of the Italian mainland uses the area around Ravenna to try to invade Venice.
Venice has commercial rivals in Genoa and Pisa, but these are on west coast of Italy. They don’t challenge Venice’s hold over the trading lanes in the Adriatic. There are three main sites of significance here that Venice needs to worry about: Zara, Bari, and Corfu.
Zara is a city on the Dalmatian coast, in lands which the Venetians claim trading rights to. It is rebellious, is a nest of pirates, and tries to play Venice, Hungary and Constantinople off against each other. When the leaders of the Fourth Crusade were unable to pay for the ships and supplies they had ordered from the Venetians, they took the sack of the city of Zara in trade, eliminating this rival.
Bari is the second most powerful trade port in the Adriatic, after Venice. It has many of the advantages of Venice, without the long sail up the Adriatic. It was the final fortress of the Byzantine empire in southern Italy before it was taken by the Normans in the 11th Century. In 1220 it is a rising power: the Holy Roman Emperor is Frederick II, king of Sicily, and this is one of his great ports. A sufficiently strong Bari could bottle Venice inside the Adriatic.
Corfu is an island of the coast of mainland Greece, at the base of the Adriatic. Like Bari, a strong fleet based there can disrupt Venice’s trade, which happens when the Normans hold it for a time. During the Xth century, the Venetians conquered it and gave it back to the Byzantine Emperors- but after the damage to the Empire in the Fourth Crusade, it was seized first by Venice, then by Venice’s rivals, the Genoese. Things get complicated here, but after about a century, Corfu becomes Venice’s main naval base – they simply can’t allow their enemies to hold it. This is a decisive shift, because Venice has previously not wanted to be an colonial power.