As mentioned in part I, & II the history of the Cairo urban area began as early as the Roman period. During the Pharaonic (3000-332 BC) and Greek (332-30 BC) eras of Egyptian history there was no town or city in the heart of what is now the Cairo area, although twenty miles to the southwest stood the great pharaonic capital of Memphis and its temples to the creatorgod Ptah, while to the northeast (located in what is now the Matariya section of Cairo) stood ancient Heliopolis, the great cult center to the sun-god Ra.
But in the Roman period there was a powerful fortified town, Babylon, on the east bank of the Nile River opposite the southern tip of Roda. This fortress was located at the strategic heart of Egypt, the dividing line between Lower (Northern) Egypt and Upper (Southern) Egypt; it also dominated the principal ford along the length of the Nile, for most caravans and armies moving east-to-west or west-to-east across Egypt skirted the edges of the Delta and made use of Roda Island as a crossing-point. The remains of the fortress of Babylon are situated in the modern Cairo region known as "Old Cairo," and it was to this site that the Arab came when they entered the country in 640 AD.
After the fall of the fortress, the Arab commander, Amr bin al-3as, chose to make the camp-city which his army had established just north of Babylon his capital. This camp-city, "al- Fustat," soon supplanted Alexandria as the demographic and economic center of Egypt, and from 700 to 1100 AD it flourished and became one of the great cities of the Mediterranean world.
But in 969 AD a dynasty centered in North Africa, the Fatimids, conquered Egypt and established a new palace city for themselves along the edge of the desert two miles northeast of Fustat; the site stood a mile-and-a-half due east of the Nile and was separated from the river by a band of swamps and lakes. The new city was "al-Qahira"Cairo: the "Victorious City"and It did not take long before the population of Fustat, attracted by the patronage available in the new royal residence, abandoned the older city and settled in the new one. By 1100 Cairo had supplanted Fustat as Egypt's most important city.
Indeed, from 1100 to 1500 Cairo can claim to have been the grandest and most splendid city in the Western world. Throughout this era its physical dimensions never exceeded five square kilometers (three square miles), and its population was never greater than 750,000 souls. However, such a total was as large or as that of any city in Europe before 1800.
Ref: Marcopedia, Wikipedia, The Ancient City