Sultan Mehmed IV who ascended the throne in 1648 inherited a vast empire which had been conquered by the sword of his ancestors and stretched over three continents. In Europe the frontier of the Ottoman Empire was a mere eighty miles from Vienna; in North Africa only Morocco did not belong to it; it included Upper Egypt and extended to Aden; the Black Sea and the Red Sea were Turkish lakes; in the east it stretched to the shores of the Caspian and of the Persian Gulf. It is impossible to give the exact figure of its population, but in the seventeenth century it amounted to approximately 25 or 30 millions. The Turks, although the dominant race, were only a minority. Probably the Muslims —Turks, Arabs, Kurds, Bosnians, Albanians, Circassians, Crimean Tatars and Turkic peoples of the Caucasus—were stronger than the Christians—Greeks, Serbs, Hungarians, Bulgars, Wallachians and Moldavians. Outstanding among the Turks of this period were the famous historian, geographer and bibliographer Kâtib Tchelebi (1609– 57) and the renowned traveller Evliya Tchelebi (1611–78), who described the cities, customs and peoples of the Ottoman Empire in his huge ten volumes. Its heart was the city of Constantinople (Istanbul), the seat of the military and administrative institutions of the empire, the centre of commerce and culture as well as amusement and pleasures. It was inhabited by more than half a million people, Muslims, Christians and Jews, all living side by side for centuries, observing their own customs. The city contained the palace of the sultans, the Serail, the splendid mosques of Sultan Ahmed, Suleymaniye, Bayezid, Selimiye and Sultan Mehmed the Conqueror, many Medresse's (colleges), libraries, public baths, hospitals, inns and food distribution centres, maintained by pious endowments.