“Doge” from the L’Archivio di Stato di Venezia: indice generale, storico, descrittivo ed analitico (1937) by Andrea da Mosto
Venice is divided into six sestiere whose origin is as old as the city of Venice itself. As far as we know, they have always been there.
Venice was an independent state for almost a millennium, but it might note have been. It all hinged on a single battle fought in the Venetian lagoon in 810.
In late Antiquity the north-east of the Italian peninsula suffered many invasions. The wars caused people to move to the safer lagoon areas, and hence to the foundation of Venice.