Eraclea (orig. Civitas Nova Heracliana, also Civitanova), founded in 640, was originally situated on a lagoon island. It was the first seat of the doges of Venice, before it moved to Metamauco.
The Birthday of Venice of is traditionally on March 25th, 421 CE, but this is far more legend than historical truth.
Venice was a republic from the beginning, but in a time when hereditary power was the norm, why did Venice become a republic and not a monarchy?