"After ten centuries of wars, defeats, and victories, the Byzantine Empire came to an end when Constantinople fell to the Ottoman Turks in May 1453. The city’s fall sent shock waves throughout Christendom. It is widely quoted as the event that marked the end of the European Middle Ages." Source: Encyclopedia Britannica.
The Byzantine Empire had been in decline for a long time before Constantinople was finally captured by Mehmed II (The Conqueror).
"The fall of Constantinople was a huge blow for the Christian world but, although Pope Nicholas V called for a crusade to regain the city for Christendom, no concerted military response was made. Now styled 'the Conqueror,' Mehmed declared the city his new capital and claimed to be the rightful successor to the Roman Empire." (Encyclopedia Britannica)
As corrupt and crazy as its rulers were, the fall of the Byzantine Empire was a calamity for the western world and Constantinople, at one point the largest and wealthiest city in Christian Europe, has remained in the hands of the Muslims ever since.
"The modern Turkish name for the city, İstanbul, derives from the Greek phrase eis tin polin (εἰς τὴν πόλιν), meaning 'into the city' or 'to the city'. This name was used in Turkish alongside Kostantiniyye, the more formal adaptation of the original Constantinople, during the period of Ottoman rule, while western languages mostly continued to refer to the city as Constantinople until the early 20th century. In 1928, the Turkish alphabet was changed from Arabic script to Latin script. After that, as part of the 1920s Turkification movement, Turkey started to urge other countries to use Turkish names for Turkish cities, instead of other transliterations to Latin script that had been used in the Ottoman times. In time the city came to be known as Istanbul and its variations in most world languages." Source: Wikipedia.
The Greek Orthodox Church still refers to the city as Constantinople, a stubborn and commendable tribute to the past. If the Islamization of Europe continues at its present pace, we may soon reach a point where all European cities have been given new names.
Related: Economic Factors in the Decline of the Byzantine Empire. (Entire article can be read online for free, but membership is required.)