Ancient Rome was a civilization that grew out of the city-state of Rome, founded on the Italian peninsula around 800 BC. During its twelve-century existence, the Roman civilization shifted from a monarchy to an oligarchic republic to a vast empire. It came to dominate Western Europe and the entire area surrounding the Mediterranean Sea through conquest and assimilation. Nonetheless, a number of factors led to the eventual decline of the Roman Empire. The western half of the empire, including Hispania, Gaul, and Italy, eventually broke into independent kingdoms in the 5th century; the eastern empire, governed from Constantinople, is usually referred to as the Byzantine Empire after 476, the traditional date for the "fall of Rome" and for the subsequent onset of the Early Middle Ages, also known as the Dark Ages.