The Ottoman Empire was not the first Islamic power to threaten the Byzantine Empire, and in fact the empire was by this point in 1453 a hollowed out husk of its former glory. The Arab caliphates tried several times to besiege the city but never succeeded in taking it. But the Ottomans were the last Islamic power to threaten the Byzantine Empire, due in large part to two things: the decrepit state of the Byzantines in 1453 and the introduction of siege cannon. Successive waves of Turkish and Mongol invasions had taken almost all of Anatolia out of Byzantine control, and the Ottomans had by this point conquered considerable portions of the empire’s Balkan territories. Constantinople itself, whose population may once have been as high as 800,000 people (500,000 is more realistic), never recovered from the Fourth Crusade’s sacking and the Black Death, and probably only had housed about 50,000 by the mid 15th century. So things were pretty rough by this point.