Sultan Mehmed II, whose great grand-father Bayezid I had previously built a fortress on the Asian side of the Bosporus called Anadolu Hisarı, now built a second castle outside the walls of Constantinople on the European side, which would increase Turkish influence on the straits. An especially relevant aspect of this fortress was its ability to prevent help from Genoese colonies on the Black Sea coast from reaching the city. This castle was called Rumeli Hisarı; Rumeli and Anadolu being the names of European and Asian portions of the Ottoman Empire, respectively. The new fortress is also known as Boğazkesen which has a dual meaning in Turkish; strait-blocker or throat-cutter, emphasizing its strategic position. The Greek name of the fortress, Laimokopia, also bears the same double-meaning.
Constantine appealed to Western Europe for help, but his request did not meet the expected attention. Ever since the mutual excommunication of the Orthodox and Roman Catholic churches in 1054, the Roman Catholic west had been trying to re-integrate the east; union had been attempted before at Lyons in 1274 and, indeed, some Paleologan emperors had been received in the Latin Church since. Emperor John VIII Palaeologus had attempted to negotiate Union with Pope Eugene IV, and the Council held in 1439 resulted in the proclamation, in Florence, of a Bull of Union. In the following years, a massive propaganda initiative was undertaken by anti-unionist forces in Constantinople and the population was in fact bitterly divided. Latent ethnic hatred between Greeks and Italians stemming from the events of 1204 and the sack of Constantinople by the Latins, also played a significant role, and finally the Union failed, greatly annoying Pope Nicholas V and the Roman Catholic church.
However, even if he had been more eager to help, Pope Nicholas V did not have the influence the Byzantines thought he had over the Western Kings and princes, and these had not the wherewithal to contribute to the effort, especially in light of France and England being weakened from the Hundred Years' War, Iberian Kingdoms being in the final part of the Reconquista, the internecine fighting in the German Principalities, and Hungary and Poland's defeat at the Battle of Varna of 1444. Although some troops did arrive from the city states of what today is the north of Italy, the Western contribution was not adequate to counterbalance the Ottoman strength.
The army defending Constantinople itself totalled about 7,000 men, 2,000 of whom were foreigners, mostly from Venice and Genoa (which had vested interests in the city); it also included a number of western adventurers. The city had about 20 km of walls (Theodosian Walls: 5.5 km; sea walls along the Golden Horn: 7 km; sea walls along the Sea of Marmara: 7.5 km), probably the strongest set of fortified walls in existence at the time. The Ottomans, on the other hand, had a larger force. It was thought to number around 100,000 men, including 20,000 Janissaries; recent estimates span between 80,000 soldiers and 5,000 Janissaries and 150,000 soldiers, including mounted troops and 6,000-10,000 Janissaries. Contemporary witnesses of the siege provide higher numbers for the military power of the sultan (Nicolò 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). Mehmed also built a fleet to besiege the city from the sea (partially manned by Greek sailors from Gallipoli). Contemporary estimates of the strength of the Ottoman fleet span between about 100 ships (Tedaldi), 145 (Barbaro), 160 (Ubertino Pusculo), 200-250 (Isidore of Kiev, Leonardo di Chio) to 430 (Sphrantzes).
On April 5, as the sultan himself arrived with his last troops, the defenders took up their positions. Constantine and his Greek troops guarded the Mesoteichon, the middle section of the land walls, where they were crossed by the river Lycus. Guistiniani was stationed to the north of the emperor, at the Charisian Gate and the Myriandrion; later during the siege, he was shifted to the Mesoteichon to join Constantine, leaving the Myriandron to the defense of the Bocchiardi brothers. Minotto and his Venetians were stationed in the Blachernae palace, together with Teodoro Caristo, the Langasco brothers, and archbishop Leonardo di Chio. To the left of the emperor, further south, were the commanders Cataneo, with Genoese troops, and Theophilus Palaeologus, who guarded the Pegae Gate with Greek soldiers. The section of the land walls from the Pegae Gate to the Golden Gate (itself guarded by a certain Genoese called Manuel) was defended by the Venetian Filippo Contarini, while Demetrius Cantacuzenus had taken position on the southernmost part of the Theodosian wall. The sea walls were manned more sparsely, with Jacobo Contarini at Stoudion, a makeshift defense force of Greek monks to his left hand, and prince Orhan at the Harbour of Eleutherius. Péré Julia was stationed at the Great Palace with Genoese troops; cardinal Isidore of Kiev guarded the tip of the peninsula near the boom. The sea walls at the southern shore of the Golden Horn were defended by Venetian and Genoese sailors under Gabriele Trevisano. Two tactical reserves were kept behind in the city, one in the Petra district just behind the land walls and one near the Church of the Holy Apostles, under the command of Lucas Notaras and Nicephorus Palaeologus, respectively. The Genoese Alviso Diedo commanded the ships in the harbour.
The Ottomans employed a Hungarian (other sources report German) engineer called Urban who was a specialist in the construction of cannons, which were still relatively new weapons. He built an enormous cannon, nearly twenty-seven feet (more than 8 m) in length and 2.5 feet (about 75 cm) in diameter, which could fire a 1200 lb (544 kg) ball as far as one mile. It was dubbed "the Basilic". Although the Byzantines also had cannons, they were much smaller and their recoil tended to damage their own walls. Urban's cannon had several drawbacks, however. It could hardly hit anything, not even as large as Constantinople; it took three hours to reload; the cannon balls were in very short supply; and the cannon collapsed under its own recoil after six weeks (this fact however is disputed, being only reported in the letter of archbishop Leonardo di Chio and the later and often unreliable Russian chronicle of Nestor Iskinder).
Another expert that was employed by the Ottomans was Ciriaco dei Pizzicolli, also known as Ciriaco of Ancona, traveller and collector of antiquities.
Siege and final assault of the city
Sultan Mehmed II planned to attack the Theodosian Walls, the intricate series of walls and ditches protecting Constantinople from an attack from the west, the only part of the city not surrounded by water. His army encamped outside the city on the Monday after Easter, April 2, 1453.
Having previously established a large foundry approximately 150 miles away Mehmed now had to undergo the painstaking process of transporting his massive pieces of artillery. The largest of these was said to have been accompanied by a crew of 90 oxen and over 400 men. Prior to the siege of Constantinople it is known that the Ottomans held the ability to cast medium-sized cannon, yet nothing near the range of some pieces they were able to put to field. Instrumental to this Ottoman advancement in arms production was a somewhat mysterious figure by the name of Urban. The master founder imediatley tried to peddle his skills to the city's invaders. . Guaranteeing Mehmed that he could cast cannons powerful enough to break down the greatest fortifications ever constructed, every resource was placed at his fingertips. In a move of unprecedented technicality, working in a makeshift foundry, Urban pushed the limits of his art and cast what was likely the largest contemporary gun ever made—27 feet long and large enough for a full grown man to crawl into. The creation of such a weapon was such a feat for its time that it took on an air of religious reverence. Urban's accomplishments in dealing with such fine tolerances on such a massive scale place his work as one of the greatest engineering feats of the time yet nothing is certainly known about his demise. For weeks Mehmed's massive cannon fired on the walls, but it was unable to sufficiently penetrate them, and due to its imprecision and extremely slow rate of reloading the Byzantines were able to repair most of the damage after each shot.
Meanwhile, Mehmed's fleet could not enter the Golden Horn due to the boom the Byzantines had laid across the entrance, and indeed had not made a real attempt at doing so; its main task was to prevent any ships from outside from entering the Golden Horn. On 20 April, however, a small flotilla of four Christian ships managed to slip in after some heavy fighting, causing embarrassment to the Sultan. To circumvent the boom, Mehmed ordered the construction of a road of greased logs across Galata on the north side of the Golden Horn, and rolled his ships across on 22 April. This succeeded in stopping the flow of supplies from Genovese ships and demoralized the Byzantine defenders. On the night of 28 April, an attempt was made to destroy the Ottoman ships already in the Golden Horn using fire ships, but the Ottomans had been warned in advance and forced the Christians to retreat with heavy losses. From then on, the defenders were forced to disperse part of their forces to the Golden Horn walls, causing defense in other walls to weaken.
The Turks made numerous frontal assaults on the wall, but were repelled with heavy losses. From mid-May to 25 May, the Ottomans sought to break through the walls by constructing underground tunnels in an effort to sap them. Many of the sappers were Serbians sent from Novo Brdo by the Serbian Despot. They were placed under the rule of Zaganos Pasha. However, the Byzantines employed an engineer named Johannes Grant (who was said to be German but was probably Scottish), who had countertunnels dug, allowing Byzantine troops to enter the tunnels and kill the Turkish workers. The Byzantines intercepted the first Serbian tunnel on the night of 16 May. Subsequent tunneling efforts were interrupted on 21, 23, and 25 May, destroying them with Greek fire and vigorous combat. On 23 May, the Byzantines captured and tortured two Turkish officers, who revealed the location of all the Turkish tunnels, which were then destroyed.
Mehmed offered to raise the siege if they gave him the city. When this was declined, Mehmed planned to overpower the walls by sheer force, knowing that the weak Byzantine defenders would be worn out before he ran out of troops.
On May 22, 1453, the moon, symbol of Constantinople, rose in dark eclipse, fulfilling a prophecy on the city's demise. Four days later, the whole city was blotted out by a thick fog, a condition unknown in that part of the world in May. When the fog lifted that evening, "flames engulfed the dome of the Hagia Sophia, and lights, too, could be seen from the walls, glimmering in the distant countryside far behind the Turkish camp (to the west),". This was interpreted by some as the Holy Spirit departing from the Cathedral.
On the morning of May 29 the attack began. The first wave of attackers, the azabs (auxiliaries), were poorly trained and equipped, and were meant only to kill as many Byzantine defenders as possible. The second assault, consisting largely of Anatolians, focused on a section of the Blachernae walls in the northwest part of the city, which had been partially damaged by the cannon. This section of the walls had been built much more recently, in the eleventh century, and was much weaker; the crusaders in 1204 had broken through the walls there. The Ottoman attackers also managed to break through, but were just as quickly pushed back out by the Byzantine defenders. The Byzantines also managed for a time to hold off the third attack by the Sultan's elite Janissaries, but the Genovese general in charge of the land troops, Giovanni Giustiniani, was grievously wounded during the attack, and his evacuation from the ramparts caused a panic in the ranks of the defenders. Sources hostile towards the Genoese (such as the Venetian Nicolò Barbaro), however, report that Giustiniani was only lightly wounded or not wounded at all, but, overwhelmed by fear, simulated the wound to abandon the battlefield, determining the fall of the city. Giustiniani was carried to Chios, where he succumbed to his wounds a few days later.
Some historians suggest that the Kerkoporta gate in the Blachernae section had been left unlocked, and the Ottomans soon discovered this mistake (there was no question of bribery or deceit by the Ottomans; the gate had simply been overlooked, probably because rubble from a cannon attack had obscured or blocked the door). The Ottomans rushed in. Constantine XI himself led the last defense of the city, and throwing aside his purple regalia, led the final charge against the oncoming Ottomans, dying in the ensuing battle in the streets, like his soldiers.
Soon the first enemy flags were seen on the walls. The Emperor and his commanders were trying frantically to rally their troops and push back the enemy. It was too late. Waves of Janissaries, followed by other regular units of the Ottoman army, were crashing throught the open Gate, mixed with fleeing and slaughtered Christian soldiers. Then the Emperor, realizing that everything was lost, removed his Imperial insignia, and followed by his cousin Theophilus Palaeologus, the Castilian Don Francisco of Toledo, and John Dalmatus, all four holding their swords, charged into the sea of the enemy soldiers, hitting left and right in a final act of defiance. They were never seen again. Now thousands of Ottoman soldiers were pouring into the city. One after the other the city Gates were opened. The Ottoman flags began appearing on the walls, on the towers, on the Palace at Blachernae. Civilians in panic were rushing to the churches. Others locked themselves in their homes, some continued fighting in the streets, crowds of Greeks and foreigners were rushing towards the port area. The allied ships were still there and began collecting refugees.