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.
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.