Grado was the seat of the Patriarch of Aquileia (later Grado) from 606 until 1172 when the patriarch moved to Venice. It was also traditionally the northernmost point of the Dogado.
Translation of the parts related to Venice of the “De Administrando Imperio” (c.950) by Constantine VII Porphyrogenitus.
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.