Most centuries can be said to have been, in one way or another, a watershed for Byzantium, but the case for the seventh century is particularly strong. At the beginning of the century, the Byzantine empire formed part of a political configuration that had been familiar for centuries: it was a world centred on the Mediterranean and bounded to the east by the Persian empire, in which most of the regions surrounding mare nostrum formed a single political entity – the Roman (or Byzantine) empire. It was a world whose basic economic unit was still the city and its hinterland; although it had lost much of its political significance, the city retained the social, economic and cultural high ground.