It is known that the Istanbul of Hellenic times was contained within the boundaries of what are now Sirkeci, the top of Alemdar Hill and Ahirkapi; in other words all the buildings of the first city were within the outer walls of the present Topkapi Palace. The city of that time was surrounded by solid walls made of hewn blocks of stone which had 27 towers and a gate on the inland side called the Thracian Gate. The Acropolis containing the ancient temples was on the hillside rising from Sarayburnu, and this had a separate wall around it. There was also at least one harbour in what is now Sirkeci.There is information available about its official buildings, temples, squares and the necropolis outside the city walls.

Until the end of the 2nd century BC Istanbul was a wealthy city within these boundaries protected by high walls. Revenues obtained from fishing, tolls paid by ships passing through the Bosphorus, and the fertility of the surrounding soil were the factors underlying its wealth in that period. In 193 AD the Roman Empire entered a period of crisis. One of the commanders who was trying to gain control during the battle for the throne that ensued with the murder of the emperor Pertinax was Pescennius Niger; he came to Istanbul and closed the road to Asia of Septimius Severus, who was engaged in battle. In 194 AD the severed head of Niger, who had been defeated by Severus, was sent to Istanbul, a city which had remained loyal to him but in spite of this the city had dared single-handedly to defy Septimius Severus, an emperor who ruled an empire stretching from Great Britain to the Gulf of Basra. This seige, which began in the winter of 193-194 AD, lasted two years. Istanbul, perhaps hoping that Severus's other rival Albinius would be victorious, stood up to the terrible seige. It was said that its people were so hungry that even human corpses were eaten. As a punishment, when Istanbul finally surrendered in 196 AD, all its warriors and administrators were slaughtered, its walls pulled down, its right to call itself a city removed and, reduced to the status of village, it was bound to Perinthos (now Marmara Ere0lisi). But Septimius Severius, who was sole ruler of the state from 193 to 211 AD, thought it was not right to leave a city with such perfect advantages as Istanbul in this position. and, according to an old story, upon the request of his son Aurelius Antoninus Caracalla, he rebuilt the city, making it even larger, and even endowed it with his name.