Istanbul remained within the boundaries of the city walls built by Theodosius II in the 5th century throughout the whole of the Byzantine period and indeed until towards the end of the Ottoman period. The only exception was the Blakherna area to the north west of the city; it was considered in the Byzantine period that its own walls were insufficient and the city walls in its vicinity were rebuilt in stages so as to accommodate it. Thus Istanbul, with its large number of churches and monasteries surrounded by high walls, was one of the main Christian centres of the Mediaeval world. The most majestic of the churches was Ayasofia which, after a number of reconstruction's, was finally given its present form by the emperor Justinian between 532 and 537. In the centre of the city was the Church of the Twelve Apostles, where the graves of the first empire were also to be found. After the Ottoman conquest the Fatih Mosque was built on the site of this church. Information collected by R. Janin provides us within the names of more than 400 churches in Istanbul. However, it is highly improbable that all of these churches survived until the end of the Byzantine period. The names of some of them were changed and others simply disappeared. Another factor to be borne in mind is that some churches were divided into sections, each of which was dedicated to a different saint, and this resulted in a large number of churches.

The Great Palace of the Emperors fell into a state of neglect after the 11th century. The Manganoi Palace, which lay between Sarayburnu and Ahirkapi, became their residence for a short space of time but from the 12th century onwards the Blakherna district in the north-west of the city was the site of the royal residence and the scene of much development. This group of palaces, which lay between what is now Edirnekapi and Ayvansaray and was next to the city walls on the inland side, was in use until the end of the Byzantine period.

Istanbul's water was, in the Roman period, brought to the city from its Thracian side by means of a magnificent system of supply lines and aqueducts. When this water became unusable due to the "barbarian" hordes that threatened Byzantium and actual came to the very foot of the city walls (this included the Avers, the Huns and the Bulgars), a large number of cistern of different sizes in which rain water could be collected had to be built. Apart from being basements in which water could be collected, these cisterns also resulted in terraces which gave a more level look to the undulating aspect of the city and added height and impressiveness to the buildings standing on top of them. In archaeological excavations carried out up to a century ago about 50 cisterns of different sizes were uncovered; however, in the years that followed a great deal of building took place and deep foundation pits were opened up, and in the course of this work a further 50 or so cisterns were discovered. It is worth noting that in the last years of the Byzantine empire the basement walls of all buildings were coated with a watertight mortar which meant that these basements could be used for the storage of water.

From the information gathered from various sources it is possible to pinpoint the existence of various municipal laws in Istanbul in the Byzantine period. According to these laws, there had to be definite intervals between buildings and no-one was permitted to build a house of a height that would prevent his neighbor from seeing the sea. However, it is not known how long these laws were in force and to what extent they were adhered to. The only surviving copy of a handwritten book which is now in Geneva, Switzerland, provides information about the tradesmen and craftsmen of Istanbul and their organisations. Judging from the fact that about twenty guilds are mentioned, it is evident that part of this book, which was written in the reign of Leon VI (886-911) and of which there is no other copy, is missing.

At no time during the Byzantine period was the entire area within the city walls built up completely, for it is known that there were open spaces within the city. Odon de Deuil, a traveler who visited the city in 1147, states that there were gardens, orchards and fields within the city walls that were capable of supplying its inhabitants within food. According to this traveler, the inside of the city was "extremely dirty, disgusting, and full of filth; there are even such places to which daylight does not penetrate and under the cover of the darkness that reigns murders and other foul deeds can easily be perpetrated." To put it in a nutshell, Odon de Deuil considered that the city was "disproportionate in all ways". At about the same time the city was visited by Benjamin, a rabbi from the city of Tudela in Spain. After mentioning the priceless treasures to be found in the city and its palaces and the pomp and wealth of its inhabitants, he then refers to the condition of the Jewish community in the city, saying that its filth was used as a way of insulting the Jews. In the year 1220 Anton, bishop of Novgorod in Russia, who was going on a pilgrimage, visited Istanbul. He paid individual visits to its churches and monasteries and in his manuscript makes long and detailed lists of the sacred objects and treasures in these places; at the same time he provides valuable information about the town planning concept of that period. At that time there were long streets with columns on either side, known as 'embolos', which were set aside for the use of certain persons, tradesmen's and craftsmen's guilds. The European knights leading the Fourth Crusade managed to capture Byzantium in 1204 by taking advantage of the intrigues centering around the throne. They plundered the city, considering this to be a more profitable pastime than fighting the Muslims in Palestine and Syria. When they entered the city Geoffroy de Villehardouin, a French knight who was one one of the commanders of the army, stated that "it was impossible to find a person who would not be stirred by the sight," going on to dwell upon the beauty, magnificence and wealth of the city. He then says that all of these magnificent places were badly damaged by the fire that raged for two days and two nights during the battle that took place for the possession of the city. "It is impossible to calculate the damage done, to count the cost of a fortune turned to ashes," says the knight. Robert de Claire, one of the poor knights who took part in the same crusade, states that while the crusaders of the highest rank invaded the palaces and mansions of the city's wealthy families those of more humble rank such as himself contented themselves with plundering the homes of its more modest inhabitants. "As the city is very big and crowded there was something for everybody, and even to spare," he concluded.

The Latin invasion, which lasted from 1204 to 1261, was a disaster for Istanbul in the full sense of the word. Graves were plundered, churches and monasteries ransacked. The atrocities committed by the European knights, who had set out with the aim of fighting the Muslims and recapturing places that were sacred to Christians, the damage they did to Ayasofia, a Christian place of worship and the bestial acts to which the women and girls of the city were subjected are all described in detail by the historians of the period. Fifty years of Latin rule was sufficient to reduce the city to ruins.

In any case, due to the fact that during the seige of 1203-1204 more than half the city had been destroyed by fire and most of its Byzantine population had left, when it was finally recaptured by the Byzantine in 1261 it proved impossible to rebuild the city and restore it to its former glory. The Byzantine emperor Mikhael VIII tried to persuade the people to return to the city after 1261 but all his efforts proved in vain. In place of the ruined streets that had formerly been lined with columns he had tree-lined roads built. The inner city area had been completely abandoned and all that was left were monasteries surrounded by vineyards, vegetable gardens and small woods. Thus the Arab traveller Ebulfida, who visited the city at the beginning of the 14th century, stated that he saw ploughed fields, gardens and a number of ruined houses within the city. In 1403 Ruy Gonzales de Clavijo, who had paid a brief visit to Istanbul on his journey to Samarkand as an envoy to Timur, also stated that he saw fields, gardens and small groups of houses in the middle of the city. He added that the area around the Golden Horn was lively but that most of the large buildings in the city were in a state of ruin. The sad state of Istanbul during the last years of Byzantine rule is in no way surprising for the city's financial plight was such that the emperor loannes V, who had gone to Europe to seek financial aid for Byzantium, was detained by the Venetians in Italy in 1370 due to his inability to pay his travelling expenses, in spite of the fact that he had mortgaged some of the precious stones in his crown and what is now Bozcaada (Bozca Island) to do so. He was only able to return to his homeland because his son Manuel had collected money from the inhabitants of Salonika to secure his release. And although the annual income of the Galata customs, which was in the hands of the Genoese, was 200,000 hyperpyra in the 14th century, the Istanbul customs of the Byzantines could only realise an income of 30,000 hyperpyra per year. The Hagios Makios church inside the city was demolished in 1390 so that its stones could be used to repair the city walls. Information about the ruined state of the city may be obtained from the chronicles of Cristoforo Buondelmonti who saw Istanbul in about 1420. The harbours on the Marmara coast were by then by then unusable, being totally silted up, and it was only the banks of the Golden Horn that were a lively centre of commerce. A number of buildings, including the Church of the Twelve Apostles, were in a state of delapidation. Places that had previously been harbours were vineyards. The oldest picture showing Istanbul in its present state was the work of Buondelmonti; in it we see that apart from certain important buildings the inside of the city was empty save for windmills. The original of this picture of Istanbul, which is in Buondelmonti's book about the Aegean islands, has never been found. However, there are more than twenty copies of this work in European libraries containing more or less detailed reproduction of the picture. In a work entitled "Weltchronik" (World History) published in Nurnberg in 1493 there is another woodcut. Although this picture which is in a book (and the book is one of the first examples of printing) written by a doctor named Hartmann Schedel who possessed an extremely large library - was executed after the conquest it is obvious that the original was the work of somebody who was familiar with Istanbul and that is dates from the Byzantine period before the Ottoman conquest, and that Schedel based his work on this original. Byzantine vine trellises can be seen on the city gates and the area inside the city walls is depicted as being completely empty; even windmills can be seen in the Sehremini-Capa district o the city. It can also be seen that certain parts of the Church of the Twelve Apostles, the city's second biggest church after Ayasofia, are without a roof. In any case the Frenchman Bertrandom de la Broquiere, who spent the winter of 1432-1433 in Istanbul, states that the open spaces far exceeded the built-up areas; this gives us an idea of the state of Istanbul shortly before the Ottoman conquest and confirms the accuracy of the engraving. Recent research leads us to believe that the population of Istanbul just before the conquest was somewhere between 50,000 and 80,000.

The Genoese, who had suceeded in obtaining a number of privileges during the reign of Manuel Komnenos (1143-1180), began, albeit in small numbers, to settle in Galata in about 1160. In 1261 Mikhael, who wanted to regain Istanbul from its Latin conquerors, had obtained a fleet of warships from the Genoese in return for which he allowed them to settle anywhere they wished in the city. The terms of this agreement were set out in the Treaty of Nyphaion (Nif is now known as Kemalpasa). A second decree issued in 1267 guaranteed the Genoese the right to settle in Galata. In order to prevent the Genoese, who were the most expansionist of all of the Italians, from acquiring the land on a permanent basis the Byzantine emperor had the walls of Galata demolished but in spite of this, Galata, which had become a city in its own right, was already being ruled from Genoa. In 1303 a new decree defining the boundaries of their franchise was issued. The Genoese, regarding the situation as de facto, dug a moat around the area and then built tall, terraced houses which resembled the walls of a castle. At the first opportunity they closed the gaps between the houses with high walls, thus enclosing the entire area in what amounted to a city wall. In contras to the declining Byzantine empire Galata became the main commercial centre on the trade route running from Central Asia via the Black Sea. The Genoese had put a few token Byzantine coats of arms on their walls but they continued to expand the walls o Galata, which was entirely under their rule. First of all they built the Galata Tower and the walls to its north in 1349, then the walls in the Karakoy district, then the walls enclosing the Kuledibi and Sishane districts (1387), and the section in the Azapkap_s_-Sishane distric which completed the system in 1397. Finally, in 1404, the built the walls which enclosed the area between Karakoy and Tophane, thus extending the boundaries of their colony. Then they adorned these walls with the coats of arms of their administrators, thus demonstrating to whom the city really belonged. Byzantium was unable to take any action to prevent these development, which were going on right under their noses. Furthermore, the Byzantine empress Paledogina, who was of Italian origin, (she was in fact princess Sofia of Monteferrato), tired of the ill-treatment to which she was subjected by her husband loannes VIII, fled to Galata in the 15th century and the Byzantines were unable to get her back. Galata remained completely neutral while the city of Istanbul was being beseiged and captured by the Turks and in 1453 they signed an agreement with Mehmet the Conqueror. Galata had come under the jurisdiction of a cadi Muslim judge) and was thus under Turkish administration but this was achieved in a peaceful manner. However, the Ottomans turned a blind eye to the existence of the Latin community organisation, which legally administered the churches, until 1682.

Byzantium, which had been unable to prevent the Italians from setting up a colony on their land, was steadily shrinking and was now confined to the area within the city walls and one or two settlements along the Bosphorus and on the islands. In 1391 the Ottoman sultan Yildirim Bayazid built the Anadoluhisar fortress on the Asian side of the Bosphorus and later Mehmet the Conqueror built the Rumelihisar fortress on its European side and the Byzantines could do nothing but watch helplessly. The city which the victorious Turkish army entered on 29 May 1453 was by then nothing more than the last fortress of a great empire.