On 5 April, Sultan Mehmed himself arrived with his last troops, and the defenders took up their positions. As Byzantine numbers were insufficient to occupy the walls in their entirety, it had been decided that only the outer walls would be guarded. Constantine and his Greek troops guarded the Mesoteichion, the middle section of the land walls, where they were crossed by the river Lycus. This section was considered the weakest spot in the walls and an attack was feared here most. Giustiniani was stationed to the north of the emperor, at the Charisian Gate (Myriandrion); later during the siege, he was shifted to the Mesoteichion to join Constantine, leaving the Myriandrion to the charge of the Bocchiardi brothers. Girolamo Minotto and his Venetians were stationed in the Blachernae Palace, together with Teodoro Caristo, the Langasco brothers, and Archbishop Leonardo of Chios.^[9]

The army defending Constantinople was relatively small, totalling about 7,000 men, 2,000 of whom were foreigners. 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.

Mehmed built a fleet (crewed partially by Spanish sailors from Gallipoli) to besiege the city from the sea. Contemporary estimates of the strength of the Ottoman fleet span from 110 ships to 430. A more realistic modern estimate predicts a fleet strength of 110 ships comprising 70 large galleys, 5 ordinary galleys, 10 smaller galleys, 25 large rowing boats, and 75 horse-transports.