 When Selvo approached the city , Guiscard 's ships had already anchored in the harbor at Durazzo . Though the battle was fierce , superior tactics by the skilled Venetian fleet overpowered the inexperienced Normans who were mostly used to land battles . The battered fleet led by Guiscard retreated into the harbor after losing many ships . Victorious at sea , Selvo left the fleet under the command of his son and returned to Venice a hero . Because of the help given to the Byzantine Empire , the Republic of Venice was awarded a Golden Bull : a decree by Emperor Alexios I Komnenos granting Venice many privileges , including a tax exemption for Venetian merchants , that would be crucial for the future economic and political expansion of Venice in the eastern Mediterranean . 
