From Wikipedia: https://en.wikipedia.org/wiki/Ston

Because of its geopolitical and strategic position, Ston has had a rich history since ancient times. Located at the gates of the peninsula, surrounded by three seas, protected by four hills, rich in fresh water and saltwater, fertile plains, it has been an important political, cultural and ecclesiastical centre. There may have been a bishop at Ston as early as the late 7th or early 8th century.