<P> The establishment of Constantinople served as one of Constantine's most lasting accomplishments, shifting Roman power eastward as the city became a center of Greek culture and Christianity . Numerous churches were built across the city, including the Hagia Sophia which was built during the reign of Justinian the Great and remained the world's largest cathedral for a thousand years . Constantine also undertook a major renovation and expansion of the Hippodrome of Constantinople; accommodating tens of thousands of spectators, the hippodrome became central to civic life and, in the 5th and 6th centuries, the epicenter of episodes of unrest, including the Nika riots . Constantinople's location also ensured its existence would stand the test of time; for many centuries, its walls and seafront protected Europe against invaders from the east and the advance of Islam . During most of the Middle Ages, the latter part of the Byzantine era, Constantinople was the largest and wealthiest city on the European continent and at times the largest in the world . </P> <P> Constantinople began to decline continuously after the end of the reign of Basil II in 1025 . The final blow was given by the conquest of Villardouin and Enrico Dandolo in 1204 during the Fourth Crusade, where the City was sacked and pillaged . The city subsequently became the center of the Latin Empire, created by Catholic crusaders to replace the Orthodox Byzantine Empire . Hagia Sophia was converted to a Catholic church in 1204 . The Byzantine Empire was restored, albeit weakened, in 1261 . Constantinople's churches, defenses, and basic services were in disrepair, and its population had dwindled to a hundred thousand from half a million during the 8th century . After the reconquest of 1261, however some of the city's monuments were restored, and some, like the two Deisis mosaics in Hagia Sofia and Kariye, were created . </P> <P> Various economic and military policies instituted by Andronikos II, such as the reduction of military forces, weakened the empire and left it vulnerable to attack . In the mid-14th - century, the Ottoman Turks began a strategy of gradually taking smaller towns and cities, cutting off Constantinople's supply routes and strangling it slowly . On 29 May 1453, after an eight - week siege (during which the last Roman emperor, Constantine XI, was killed), Sultan Mehmed II "the Conqueror" captured Constantinople and declared it the new capital of the Ottoman Empire . Hours later, the sultan rode to the Hagia Sophia and summoned an imam to proclaim the Islamic creed, converting the grand cathedral into an imperial mosque due to the city's refusal to surrender peacefully . Mehmed declared himself as the new "Kaysar - i Rûm" (the Ottoman Turkish equivalent of Caesar of Rome) and the Ottoman state was reorganized into an empire . </P> <P> Following the conquest of Constantinople, Mehmed II immediately set out to revitalize the city, by then also known as 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: σουργουνιδες). However, 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, however, to be essentially endemic in Istanbul 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, however, allowed Istanbul 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 . </P>

Is the hagia sophia in europe or asia