Although films concerning wars are not uncommon in cinema history, depictions of full-scale battles were rather sparse until recent years. Especially since the 2000’s, however, large battles and sieges have become more commonplace, probably both because of technical advances (the large numbers of soldiers in modern war-movies are computer generated), and the ever-expanding budgets of blockbuster Hollywood films. Examples include the huge battles seen in the Lord of the Rings trilogy (2001 – 2003), but also historical battles, such as in Troy (2004, the Trojan war), Kingdom of Heaven (2005, the siege of Jerusalem in 1187) and 300 (2007, the battle of Thermopylae in 480 BCE). Historical fiction can leave a strong impression, because the consumer knows that what he reads or sees really happened, and possibly had strong, direct consequences that are still visible today. The story behind Kingdom of Heaven reveals the seed of some of the conflict we see in the Middle East today, and some historians claim that we can thank the effort of the 300 Spartans (and the other thousand or so Greeks not portrayed in the film) for the existence of Western civilization today. If a historical film also contains its share of drama, suspense and Hollywood clichés, it has the potential to be quite a success. I believe there is one such historical battle which contains all the necessary ingredients to become a successful epic film; the siege of Constantinople in 1453 by the Ottoman Empire. Not only was this in absolute terms one of the largest and most extensive city sieges undertaken until modern times, but the eventual fall of Constantinople is a momentous event in European, even in world, history, and is by many historians regarded as the end of the thousand year long Middle Ages; not to mention it was also the end of one of the most influential and enduring political entities in history, the Roman Empire. Add to this all the suspenseful events leading up to and during the siege, and a person gallery closely resembling that of a Hollywood movie, as recorded in letters and diaries from the Byzantine side inside the walls, and it is quite baffling that there exist to date no films (to my knowledge) concerning this, by any definition, truly epic battle.
The city of Constantinople (modern day Istanbul) was founded by Greek colonists under the name of Byzantium in the 7th century BCE, but was with time incorporated with the rest of the Greek cities into the Macedonian, and later the Roman Empire. By the 4th century CE, the Roman Empire was declining in the west, and its new important frontiers and trade relations were in the east. The Roman Emperor Constantine I (lived from 272 to 337), the first Christian emperor, also known as Constantine the Great and widely regarded as one of the greatest of all the Roman Emperors, moved the capital of the Empire from Rome to the (by then) much more central port city of Byzantine, rebuilt it and possibly had it renamed to Nova Roma (New Rome) or something similar. However, it quickly became popularly known as Constantinople, «City of Constantine». By chance, the last Roman Emperor’s name was also Constantine (XI), and he would die a true (Hollywood) hero’s death in the city built by the first Emperor Constantine, more than a thousand years later.
Constantinople thrived, and so did the Empire, although the western part of it, including Rome, gradually fell out of its hands; its new power base was the Balkans and Anatolia. Confusingly, modern histories often refer to this later period of the Empire as the Byzantine Empire (alternatively the Eastern Roman Empire), even though this was the name of the city before it became the capital; but to people at the time, it was simply the continuation of the already long-enduring Roman Empire (and I too shall refer to Constantine XI’s empire as this). However, both the emperor and the population of Constantinople and most of the Empire’s territories spoke Greek, and the culture was Hellenic, albeit with strong Roman influences and traditions. Strong walls were built around the city, reinforced and added to many times, and although Constantinople was besieged many times in its history, it was only successfully entered once before 1453; by the soldiers of the Fourth Crusade in 1204. The crusade was originally sent to liberate Jerusalem from Muslim rule, but the Roman Catholic crusaders unexpectedly ended up invading the great bastion of the Greek Orthodox Church, Constantinople, instead, entering through the sea walls along the Golden Horn. They subsequently completely sacked Constantinople, gathering a legendary amount of loot, and burned down most of the city. The Emperor escaped, however, setting up an exile government, and eventually reconquered the city from the disorganized Latin crusaders. Although much was rebuilt, and the sea walls strengthened, the city never quite recovered from this blow, and the relations between Constantinople and Rome understandably became quite sour. From now on, moreover, the entire Empire was in a slow decline. Efforts were made to patch up the schism between the east and west, to unite against the new Balkan nations and the growing Muslim threat from the east. However, there was much opposition both in Rome and Constantinople to such plans, and not much progress was made.
By the middle of the 15th century, Constantinople had been Imperial capital for more than a thousand years, but the Empire itself seemed to be in its last throes. Apart from the Peloponnese peninsula on the other side of the Aegean Sea, it only held Constantinople and its suburbs. Its power was now largely an illusory one based on prestige, and the Imperial army could no longer fight any wars; especially now that the Turks of the Ottoman Empire had taken almost all its possessions in Anatolia to the east, and large parts of the Balkans to the west, and therefore completely surrounded Constantinople. When the 19 year old Mehmed II was throned as the leader of the Ottomans, many were relieved, hoping that the young sultan would not present much of a threat. Mehmed immediately set his sights on the old Imperial capital, however, and against his advisers’ wishes started preparations for a huge siege.
The above historical background is relevant for the story of the siege, and so a film could present some of it (depending on which events during the siege it emphasizes) through a quick narration, or perhaps flashbacks, but the main story would start now, sometime between 1451 and 1453, the year of the siege. Our main character and hero is Constantine XI, emperor in the city founded by Constantine I (though they were of different dynasties, and so he was not an actual descendant). Most accounts describe him as an able and calm emperor, quite fitting for his name, which is connected to the English “constant”, but also can be translated from Latin as “steadfast”. When Mehmed started construction of a fortress on the Bosporus north of Constantinople in 1451, in order to control travel between the Black Sea and Constantinople, Constantine understood that the Sultan’s plans were to conquer Constantinople, and with it the remains of the Roman Empire. Constantine proceeded with asking the western Christian nations for assistance, eventually promising union between the two churches more or less on the Catholics’ terms. A half-hearted Catholic-Orthodox union was proclaimed in December 1452, and the Pope agreed to help assemble a fleet to assist Constantinople. It took a long time for the various Italian city-states asked to reach an agreement on the details of any expedition, however, and the only sizeable relieving fleet that was eventually sent, by Venice, did not set sail until late April, long after the siege had started. This would be a good occasion for a film maker to show the contrasts between the bureaucratic, constantly bickering Italians, and our valiant and flexible Greek heroes in Constantinople. While the Vatican, though militarily quite powerless, at least wholly supported Constantinople in their struggle, many in the other Italian merchant city-states probably had mixed feelings, as a large portion of their income already came from trade with the Ottomans. Although several of their trading colonies (like the Genoese Pera, on the other side of the Golden Horn) were enjoying Imperial protection, they were reluctant to directly confront the Ottomans, fearing retributions. Most people outside of Constantinople were also largely ignorant of the urgency of the threat, unable to imagine the more than thousand year old Imperial capital, well-fortified, suddenly falling into Muslim hands.
Constantine, therefore, largely had to make due with the forces he already had – about 7000 soldiers, guarding what had once been one of the largest cities in Europe, although now only housing around 80.000 citizens. He was facing one of the strongest empires in the world at the time, capable of supporting armies of hundreds of thousands of soldiers, fanatical Muslims who saw Constantinople as their last real obstacle to freely entering Christian Europe; led by Mehmed II, in Western accounts often described as a fanatical and irrational sultan. The emperor was advised that his city was lost, and that he should leave it and create a Roman government in exile, as successfully executed by the emperor during the fall of Constantinople to the crusaders in 1204. Constantine, however, had no illusions that he could present a threat to the Ottoman Empire without his capital, once the most magnificent city in Europe. He decided that this time, it was all or nothing; he would rescue Constantinople, or he would fall with it, and the Empire with him.
Although grim, the situation was not completely hopeless. The defenders were few in numbers, but the defensive constructions in and around Constantinople were formidable, and the city was called by some the best-defended city in Europe. Constantinople borders to the Sea of Marmara, which connects the Dardanelles (and thereby the Mediterranean) with the Bosporus (and thereby the Black Sea), to the east and south, and the bay called the Golden Horn to the north. The sea walls along the Sea of Marmara were strong, and made a naval assault from there extremely difficult. The walls along the Golden Horn, although reinforced, were not as secure, but Constantine ordered the entrance of the bay closed with a huge chain, or boom, which was virtually indestructible. In addition, the city was surrounded by the so-called Theodosian land walls, built by Emperor Theodosius II some 1000 years earlier, and reinforced many times since. Put together, these walls constituted one of the strongest fortifications built by man in pre-gunpowder times. The Imperial navy consisted of few, but well-armed ships with experienced crews. The Ottomans, like most empires originating from Asian nomadic tribes, had little experience on and interest in the water, but quickly assembled a rather large fleet in order to blockade the port and hinder outside forces to relieve Constantinople. Their seamanship was, however, far inferior to that of the Imperial navy. All put together, although Constantine could not hope to crush the attacking forces, he prepared a defence to hold out long enough for help from the west to arrive. A ship was sent to forage for supplies in Sicily, and to again plead for assistance. A few months before the siege began, in January 1453, our second protagonist entered the city; Giovanni Giustiniani, from the Italian merchant city-state of Genoa, arrived with a force of 700 well-armed and trained men. Although the Genoese senate was still discussing what kind of help to send to Constantinople, Giustiniani had used his own private funds to raise a small force and help the city. An expert in the defence of walled cities, he was warmly welcomed by the Emperor and immediately set to command the defence of Constantinople. Shortly before the siege began, on 3 April, a small ship with a crew of twelve was sent towards the Dardanelles to look for the relieving western fleet.
By April 1453, Ottoman troops were pouring into the area around Constantinople, and the harbour was effectively blocked by more than a hundred Ottoman ships; the twenty or so Imperial and Venetian ships left were in the safety behind the chain in the Golden Horn. The actual number of Ottomans participating in the siege were at the time estimated to be anything from 160.000 to 300.000, although modern estimates put it closer to 80.000. Any number would allow a film maker to create a true (computer animated) “Muslim horde”, moving (and later attacking) in a disorganized and barbaric fashion, in accordance to all available accounts. Accompanying the army was also Mehmed’s “secret weapon”; a giant cannon reportedly pulled by 60 oxen and guarded and maintained by 400 men, transported 240 kilometres to the site of the siege. It was designed and created by a Hungarian engineer known as Orban. He is supposed to have a first gone to Constantinople to offer his services, but when they had neither the funds nor materials he required, he instead turned to Mehmed, promising he could build a cannon capable of bringing down the walls of Constantinople. The result was the largest cannon ever produced until then, and one of the first really effective against strong masonry. Sieges until now had largely been a battle of attrition between the attacker and the defender, the side with the least supplies eventually surrendering. The installation and commencement of firing of the new Ottoman cannon quite literally rang in a new era of European siege warfare, and marked the end of pre-gunpowder warfare in Europe: gunpowder weapons were supplanting swords and bows, and fortifications had to be rebuilt to stand a chance in a siege.
When the several hundred kilograms heavy cannon balls started pounding on the Theodosian walls in early April, they immediately started doing serious damage. However, this early heavy cannon required hours to reload and prepare, and the defenders managed to repair any breaches that were made between the shots, or at night. Although the defending army did not number more than 7000, its regular inhabitants, knowing this was a battle of survival, were also an indispensable support, supplying the soldiers at the walls and repairing any breaches made by the cannon balls. Several attempts to scale the walls with ladders were made by large numbers of Ottoman irregulars, but the Ottoman attackers were easily cut down. Unable to make any advances without a proper breach, Mehmed grew impatient after a few weeks of bombardment, and ordered miners to commence operations in order to weaken the walls from below. The following events could be a decent side-story to a filmization of the siege. The Ottoman sappers, though numerous, were inexperienced, and progress was slow. In Constantinople, moreover, there were experienced miners, and they started counter-mining operations. Spotting the Ottoman mine entrances from the walls, they managed to dig into their mines before they reached the walls, and subsequently sent soldiers in to kill the miners, burn the tunnel supports, or flush them with water. By late May, tenths of tunnels had been dug and destroyed, and Mehmed ordered the operations to stop. Nothing had been accomplished, apart from wearing out the defenders.
That, however, was not a negligible victory. Although almost no one in Constantinople had been killed during the siege, not even during the scaling attempts, many had been wounded from missiles, and everyone were exhausted. Food supplies were low, and Constantine knew they could not sustain a siege forever. Spirits were therefore raised when a group of four western galleys were spotted in the Sea of Marmara on 20 April. It turned out to be the Imperial galley sent a while earlier to forage for supplies, now full of food, followed by three Genoan ships, filled with well-armed soldiers. The ships were sailing straight for the Golden Horn, the wind blowing them faster than the Ottoman ships could pursue them. Suddenly, though, the wind changed direction, and the three ships were stuck, unable to move. The armada of Ottoman ships encircled them, and started attacking. The much more experienced Imperial and Genoese soldiers, however, managed to push back every wave of Ottomans trying to board them, and set fire to the enemy ships with the famous inflammable mixture “Greek fire”. But with every repulsed Ottoman ship, several new ones were ready to replace it. Finally the wind started blowing in the right direction again, and the three ships managed to wrestle themselves out of the myriad of enemy ships, and barely slipped into the safety behind the chain of the Golden Horn. Mehmed was understandably furious that his comparably huge fleet had not managed to stop even one of the three ships from penetrating his blockade and reaching the city, and exiled his top admiral. Many of the western soldiers had been injured, but few had been killed, and the city received a much-needed supply of men, weapons and food.
This, however, was one of the last good news to reach Constantine. Even with the new contingent, his defence could do little more than wait for a real relieving fleet from the west. Mehmed, too, was growing impatient, and especially of his fruitless efforts to enter the Golden Horn. The chain stretched over the strait was too strong to break by ramming, and Constantinople was therefore able to freely communicate with Pera, the Genoese colony on the other side of the bay, to which the other end of the chain was fastened. Although Pera was officially neutral in the conflict, it was in their interest that Constantinople should be the victor in the conflict, and both information and supplies flowed between the two cities. As long as the communication was done in relative secrecy, Mehmed was reluctant to attack the city directly, as that might provoke the Genoese to answer much more strongly militarily than they had until now. By late April, he therefore launched a new plan to get into the Golden Horn (and the careful Hollywood director would include that the idea quite possibly came from an Italian, and not a Muslim). On April 22, the few men on the sea walls facing the Golden Horn witnessed a truly fearful sight: in a show of pure and raw strength, thousands of Ottomans were pulling the better part of the Ottoman armada, tenths of huge ships, down the ridge towards the bay, one by one. The soldiers of Constantinople could do nothing but watch as the Ottomans set their ships back on water, this time on the inside of the bay, and suddenly the entire sea wall was in danger. Although the Greek and Italian ships in the bay managed to avoid battle and hide near the boom, where they were better covered from the walls, Giustiniani, the Genoese commander of the defence, now had to distribute soldiers from other parts of the city to defend this long sea wall as well. A small fleet was assembled from Constantinople as a response to the Ottoman move, aiming for a surprise attack on the remaining Ottoman ships in the Sea of Marmora. However, the usual resentment between the Venetians and Genoese, and between both of them and the Greeks, led to demands for postponement from the Genoese, and the secret was eventually leaked to the Ottomans, probably through the Genoese in Pera. The Ottomans were prepared when the expedition finally came, and it was a disaster: ships were lost, men were killed, morale in Constantinople dropped, and the resentment and bickering among the Italians soared. Both Constantine and Giustiniani had to intervene to ask the leaders of the respective camps to uphold a modicum of cooperation, for the sake of the effective defence of the city.
But the worst news was yet to reach them. On 23 May, a ship was sighted in the Sea of Marmara. The Ottoman ships tried to stop it, but it narrowly escaped through the chain into the Golden Horn, docked, and its captain was received directly by the Emperor. Everyone in Constantinople were full of hope that this was the first ship in a relieving fleet from Italy, arriving at the last minute to save them. Constantine quickly understood, however, that this was the same ship he had sent to search for such a party well over a month earlier. The ship had travelled to the Dardanelles and searched for any signs of an incoming fleet, but had found none. (A Venetian fleet was on its way and would have reached them a while later, but it arrived too late.) When they understood their expedition had been in vain, the crew of 12 had almost unanimously decided it was their duty to return to Constantinople and inform of this. This essentially amounted to reporting to the Emperor that his city was doomed, and their decision to return, as pointed out by the only member of the crew that was against it, more or less sealed their own fates as well. According to witness accounts, the Emperor wept at the news.
Whether the city was now truly beyond rescue is a matter of dispute among historians, though from surviving accounts it appears that this was a generally held belief. In either case, there does not appear to have been much defeatism in the city, despite the circumstances. Although Mehmed suggested terms of surrender, allowing free passage out of the city to the citizens if they were to hand their city over, few people were in favour of such a solution. After two months of fighting and destruction, they did not want to give it up just like that. Either way, few seem to have had much faith in Mehmed’s promises, although such promises were traditionally upheld by Muslim conquerors. The offer was declined, and Constantine continued to do everything in his power to strengthen his defences.
By late May, although the defenders were fatigued and their defences weakened, not much actual progress had been made by the Ottomans. The Golden Horn was in their hands, but they were hardly any closer of getting into the actual city. Mehmed knew he could not keep up the siege forever, and started planning the final offensive. The siege cannons aimed concentrated fire on the places of the Theodosian walls that were considered especially weak, and after a while of continuous fire, on May 29 the army was sent into the breaches. The first waves were irregular Anatolian Muslims and some Balkan Christians, and their lack of experience and weapons made them an easy match for the defending Imperial soldiers. The numeric superiority of the Ottomans made little difference in the small breaches, and wave after wave was thrown back at every point. Giustiniani was everywhere, redistributing soldiers to where they were most needed and keeping up morale. The emperor himself also inspected the defence in full armour, at times participating directly in the fighting. Finally the attacks dried up, and the Imperial soldiers could pull back to their posts, while the breaches were quickly patched up. But the soldiers were exhausted. Not only had they been starving and set to laborious reparations on the wall every night for weeks, but they had been fighting an exhausting hand-to-hand fight for four hours. And just then, when they thought they could take a rest, Mehmed sent in his trump card.
The Janissaries were the Sultan’s elite force. Taken from their Christian parents in occupied lands when they were young, they were brought up to be loyal Muslim soldiers since childhood, and their discipline and fighting skills were of an altogether different sort than that of the Anatolian irregulars (a fact which would make for yet another subtle reference to Christian peoples’ superiority in a Hollywood filmization, but this time with racial undertones). The thousands of Janissaries were now sent towards the walls, moving orderly and surely forward, in contrast to the earlier irregular attackers. Holding them back proved difficult for the exhausted imperial soldiers, and the fighting at every breach was fierce. The Ottomans even sent forward a large wooden siege tower, which managed to set some men on the walls, but was successfully destroyed by the defenders.
Exactly what went wrong where is not known for certain. Some accounts tell of a small, forgotten gate by the harbour that was left unlocked, and through which a small force of Ottomans managed to get in. Perhaps the defenders at some part of the wall were simply overcome by the sheer force of the Janissaries. Whatever happened, the accounts seem to agree on that the next catastrophic event, coming at a crucial point in the battle, finally and decidedly sealed the fate of the city. Giustiniani, who had been running untiringly up and down the walls all day, cheering up the defenders where he could and reinforcing were necessary, was hit by a stray crossbow bolt (for Ottoman bolts tended to be stray bolts, as opposed to their well-aimed Christian counterparts). It was a serious wound, eventually mortal, and he fell down. Perhaps the morale of his soldiers might still have been rescued had he stayed on the walls, but the tired and exhausted man, knowing he was dying, asked to be carried off the battlefield, and ferried to safety. His commands were obeyed, and he escaped in a Genoese boat to the Genoese island of Chios, to die a few days later. The news of their commander’s fate, however, quickly spread among the troops, and the defence collapsed. Suddenly Ottoman Janissaries and irregulars were pouring through and over the outer walls of the city, and when the retreating Imperial soldiers forgot to lock the gates of the inner walls after them, Ottoman soldiers finally flowed into the Imperial capital. They slashed down the fleeing Imperial soldiers, and any other human beings they saw for that matter; men, women and children. Looting of the city began even before the defenders had been properly crushed; Mehmed had promised every soldier a fair share of whatever was looted, and every soldier was eager to get their hands on as much as possible.
Watching the havoc, Emperor Constantine probably understood the consequences of what he saw better than anyone. Having prepared and maintained the defence of the city for months, he had known for a long time, with more and more certainty, that this was the more than likely outcome. Yet he had not escaped as his predecessors had, he had not surrendered as many other cities had. He had been steadfast as a rock, as his name implied, and had made no compromises. He would make none now either, to the very last. He ripped his royal ornaments off his armour, and as a normal soldier threw himself into the battle, as a final, defiant act against the flow of history. He was killed as an anonymous soldier, his body never identified; and when he fell, on 29 May 1453, he fell together with the thousand year old capital, whose name he shared; he fell together with the 1500 year old Roman Empire; he fell together with 2200 years of Roman identity; he fell together with the definite hegemony of Christianity in the Balkans and Anatolia; and in many ways with the entire Ancient world of Europe.
With the fall of the emperor, our storyline is more or less at its end. The camera can move upwards, showing from above the total and indiscriminate brutality of the plundering Muslims, killing women and children, desecrating churches, and burning the houses they have emptied of treasures. If the film had a narrator, perhaps the director can have him escape on one of the Ottoman ships left unguarded when the greedy Muslim sailors wanted to take part in the sack; and show their relative easy escape as most of the Ottoman fleet were preoccupied with looting rather than stopping fleeing Christians. He can then safely ignore the part where Mehmed sent Janissary soldiers to defend specific churches and towns within the walls who had surrendered to him, from looting; when he stopped the looting after a day, instead of letting it last for the traditional three days; and then proceeded with giving asylum to the surviving Christians in the city (only a fraction were killed in the siege and subsequent looting), and left many of their churches to their own disposal. He then made Constantinople his empire’s capital, rebuilt it with new universities to which he invited both eastern and western scholars who would preserve knowledge of the classical world until modernity, and made it yet again one of the most magnificent cities in the world; incorporated the old Imperial administration into his Ottoman administration, reformed the Ottoman bureaucracy, let the Orthodox church continue its functions; and in general turned out to be quite a tolerable emperor to live under, even for the Christians; certainly preferable to most of the following Ottoman emperors, who would be less forgiving to their Christian subjects. Constantinople in fact received lighter treatment than most other cities that resisted Ottoman forces, and there is reason to believe that Mehmed would have held his promises had Constantine surrendered his city.
But a film maker can pick and choose, and for the siege of Constantinople there is much to pick from. The above narrative only includes the main events of the siege, and all of those also have several versions. The fall of the great city came as a shock to Europe, and was a scarring event for all the surviving participants. As a result, a large number of memoirs, letters and other texts from Italians who managed to return home, or Greeks who managed to escape to safety in Europe, have survived until today, narrating the story in considerable detail. The exact manner of Constantine’s fall, for instance, is not known for certain (and it is for me unclear exactly who lived to tell of his final heroic act); but his body was probably never found, much to the aggravation of Mehmed, and it is completely possible (perhaps even likely) that the story is true. Moreover, Muslim accounts of the siege are much more sparse. Without better sources of information, many such stories remain in the “official” history of the siege, as told by even modern history books, and it would be completely legitimate to present them as true in a film.
As has been noted already, the story is a strong one not just for its dramatic content and huge scale (though Christian deaths were comparably small, perhaps 4000, Ottomans deaths, although unknown, were undoubtedly much larger), but also for its lasting impact and consequences, visible even today. Greek and Italian scholars escaping to Europe are said to have fuelled to the Renaissance. Although the Islamisation of Anatolia and the Balkans had started much earlier, and would arguably not have been slowed down even if Constantinople had been successfully defended, the final fall of the Roman Empire decidedly marked the end of western hegemony in the area. The Christian states of the Balkans and Eastern Europe would fall under Ottoman rule one by one in the following decades, and Christian Europe would have a constant threat from the east to worry about for centuries to come. Greek communities, surviving from the days of Aleksander the Great or even earlier, would live on for many more centuries; but they were gradually more and more repressed, until the early 20th century, when an increasingly nationalist Ottoman empire organized the Greek genocide, putting perhaps a million Greeks to death. Almost the entire remainder of people with Greek heritage in the Ottoman empire (who were until then still largely living in Greek villages speaking Greek with other Greeks) were traded off for Muslims living in Greece in 1923, in one of the largest forced migrations in human history. The conflicts between Muslims and Christians in the Balkans, that continues to this day, are also a direct consequence of the influx of Muslim settlers, especially after the fall of Constantinople.
A more visible and less violent artefact of the battle, is the Hagia Sophia. Built in in the 6th century and one of the greatest accomplishments of Christian architecture, Mehmed II did not have it demolished as a centre of infidel worship. Instead, he had it turned into a mosque, further improving the structure, and it has been maintained until today, from 1931 as a museum.
This summary of the Siege of Constantinople is based on the books Lost to the West: The Forgotten Byzantine Empire That Rescued Western Civilization (2009) by Lars Brownworth and The Fall of Constantinople 1453 (1965) by Steven Runciman, and various Wikipedia articles (mainly http://en.wikipedia.org/wiki/Fall_of_constantinople).