 Complete the following phrase:  In 840, Duke Sergius I made the succession to the duchy hereditary, and thenceforth Naples was de facto independent. In this age, the city was mainly a military centre, ruled by an aristocracy of warriors and landowners, even though it had been compelled to surrender to the neighbouring Lombards much of its inland territory. Naples was not a merchant city as other Campanian sea cities like Amalfi and Gaeta, but had a respectable fleet who took part in the 