Constantinople was founded by Constantine to serve as a new imperial capital for the Roman Empire, and it would remain one of the great cities in Europe across the Middle Ages, long after the Western Half of the Roman Empire fell. Constantinople was the center of the Byzantine Empire, and even as the empire itself disintegrated, the city of Constantinople continued to endure until the Ottoman conquest of 1453.

This was one of the defining moments of Early Modern History. Constantinople was converted into a new Ottoman capital, Istanbul. The final conquest of the Byzantine Empire did much to consolidate the Ottoman Empire, and set the stage for later expansion further into Europe. Throughout the Early Modern Era, the Ottomans would represent a critical force in European politics.

With the loss of Constantinople, Christendom lost one of its critical pillars (especially within the Eastern Orthodox tradition). Furthermore, the fall of Constantinople allowed the Ottomans to control trade between Europe and Asia. Finally, this moment contributed greatly to the Renaissance, as many of Constantinople's Greek scholars fled west, bringing with them their expertise, as well as access to classical texts that had been lost to Western Europe.

In short, the conquest of Constantinople was one of the critical moments of Early Modern history. Lasting effects can be observed, both for the Ottomans as well as for the rest of Europe.

The capture of Constantinople by the Ottoman Empire was significant for both the Turks and for the Europeans because it represented a major defeat for the forces of Christianity and a major triumph for those of Islam.

Constantinopole (today's Istanbul) had been the capital of the Byzantine Empire, established as such by the Roman emperor Constantine I. The city represented both the eastern reaches of the Roman Empire and the center of Eastern Orthodox Christianity. In 1453, it fell to the Islamic forces of the Ottoman Empire, which converted the dominant religion from Christianity to Islam and made this famous city its capital. While the Ottomans respected the sanctity of the city's existing Christian institutions and allowed for the continued practice of Christianity, the loss of Constantinople remained historically significant for both the Turks and the Europeans.

Christianity had already been riven with divisions, notably the establishment of an Eastern Orthodox Church that competed with the Catholic Church across Western Europe. The latitude to practice their chosen faith made the situation of Christians living under the Ottomans more tolerable than would have been the case if they had fled to the West and the oppressive yoke of the Roman Catholic Church.

The tolerance practiced by the Ottoman Empire toward non-Islamic religions was a defining characteristic of that empire and stood it in stark contrast to the Islamic tide that had swept across the Arabian Peninsula.

The Fall of Constantinople was a significant event in world history. By conquering Constantinople, the Ottomans expanded their territory and grew their empire, while the Romans saw the decline of the Byzantine Empire.

The capture of Constantinople was important for the Ottomans because the city was highly fortified, and it provided an opportunity for the young Sultan, Mehmed the Conqueror, to test his military skills and strategies against one of the most powerful empires of his time.

The Sultan made Constantinople his new capital, which also served as a base for more forays into Europe. The Europeans, on the other hand, were forced to review their city defenses because the siege by the Ottomans proved that even the most fortified walls could be breached.

Fall of the capital marked the end of the Eastern Roman Empire and a major blow to Christendom in the east.

Whenever there is a big shift of power through conquest, there is significance. 

First, when the Ottoman Turks took over Constantinople in 1453, the Eastern Empire of Rome fell. This was the end of something incredibly significant. What stood from 753 B.C. (traditional date) was now over. This was a huge psychological blow to Europe. 

Second, from a religious point of view, there was a seismic shift. Constantinople from its beginning was a major Christian city. It produced great Christian traditions, such as icons, and theology. Now that the city was in the hands of the Ottomans, it ceased to be a Christian city. Islam would eventually take over. Even today, Islam is the dominant religion in Turkey. This point cannot be emphasized enough. 

Third, many intellectuals moved from Constantinople to Europe, which helped start the Renaissance and humanist movements. These movements would later help fuel the Protestant Reformation. 

Fourth, great animosity was created among Greeks and Turks, which would last. 

The capture of Constantinople by the Ottoman Turks in 1453 was significant for a number of reasons.  First, the capture of the city made the Ottomans the most important power in southeastern Europe and the eastern Mediterranean and began a long period of expansion for the Empire.  It also ended the Eastern Roman Empire.  Next, many Greek scholars fled to Europe after the fall of the city, which helped fuel the birth of the Renaissance and helped bring to an end the Middle Ages.  Finally, the capture of the city cut off an important trade route to the east.  This forced Europeans to begin thinking about a sea route to China and the east, sparking new ideas in seafaring and navigation and  helping to begin the Age of Exploration.