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Strength

The army defending Constantinople was relatively small, totalling about 7,000 men, 2,000 of whom were foreigners. The population decline also had a huge impact upon the Constantinople's defense capabilities. At the end of March 1453, emperor Constantine XI ordered a census of districts to record how many able-bodied men were in the city and whatever weapons each possessed for defense. George Sphrantzes, the faithful chancellor of the last emperor, recorded that "in spite of the great size of our city, our defenders amounted to 4,773 Greeks, as well as just 200 foreigners". In addition there were volunteers from outside, the "Genoese, Venetians and those who came secretly from Galata to help the defense", who numbered "hardly as many as three thousand", amounting to something under 8,000 men in total to defend a perimeter wall of twelve miles. At the onset of the siege, probably fewer than 50,000 people were living within the walls, including the refugees from the surrounding area. Turkish commander Dorgano, who was in Constantinople working for the Emperor, was also guarding one of the quarters of the city on the seaward side with the Turks in his pay. These Turks kept loyal to the Emperor and perished in the ensuing battle. The defending army's Genoese corps were well trained and equipped, while the rest of the army consisted of small numbers of well-trained soldiers, armed civilians, sailors and volunteer forces from foreign communities, and finally monks. The garrison used a few small-calibre artillery pieces, which in the end proved ineffective. The rest of the citizens repaired walls, stood guard on observation posts, collected and distributed food provisions, and collected gold and silver objects from churches to melt down into coins to pay the foreign soldiers.

The Ottomans had a much larger force. Recent studies and Ottoman archival data state that there were some 50,000–80,000 Ottoman soldiers, including between 5,000 and 10,000 Janissaries, 70 cannons, and an elite infantry corps, and thousands of Christian troops, notably 1,500 Serbian cavalry that Đurađ Branković was forced to supply as part of his obligation to the Ottoman sultan — just a few months before, Branković had supplied the money for the reconstruction of the walls of Constantinople. Contemporaneous Western witnesses of the siege, who tend to exaggerate the military power of the Sultan, provide disparate and higher numbers ranging from 160,000 to 300,000 (Niccolò Barbaro: 160,000; the Florentine merchant Jacopo Tedaldi and the Great Logothete George Sphrantzes: 200,000; the Cardinal Isidore of Kiev and the Archbishop of Mytilene Leonardo di Chio: 300,000).