The Mediterranean was the centre of the ancient, byzantine and medieval Western worlds. It was the seat of some of the greatest empires and civilisations in history, and, by joining three continents, played a strategic role in their development. Emperors and republics, popes and crusades, caliphates and metropoles all flourished around its shores. So too did the greatest cities of this long period: Rome, Constantinople, Cairo and C rdoba jostled for supremacy in a rapidly changing world.