Task: Question Answering
Input (English): Passage: Following the conquest of Constantinople, Mehmed II immediately set out to revitalize the city, by then sometimes called Istanbul. He urged the return of those who had fled the city during the siege, and resettled Muslims, Jews, and Christians from other parts of Anatolia. He demanded that five thousand households needed to be transferred to Constantinople by September. From all over the Islamic empire, prisoners of war and deported people were sent to the city: these people were called "Sürgün" in Turkish (Greek: σουργούνιδες). Many people escaped again from the city, and there were several outbreaks of plague, so that in 1459 Mehmet allowed the deported Greeks to come back to the city. He also invited people from all over Europe to his capital, creating a cosmopolitan society that persisted through much of the Ottoman period. Plague continued to be essentially endemic in Constantinople for the rest of the century, as it had been from 1520, with a few years of respite between 1529 and 1533, 1549 and 1552, and from 1567 to 1570; epidemics originating in the West and in the Hejaz and southern Russia. Population growth in Anatolia allowed Constantinople to replace its losses and maintain its population of around 500,000 inhabitants down to 1800. Mehmed II also repaired the city's damaged infrastructure, including the whole water system, began to build the Grand Bazaar, and constructed Topkapı Palace, the sultan's official residence. With the transfer of the capital from Edirne (formerly Adrianople) to Constantinople, the new state was declared as the successor and continuation of the Roman Empire. 
Question: What was Instanbul also called?
Output (English): Constantinople.