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 reconstructions, 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 with 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 actually came to the very foot of the city walls (this included the Avars, the Huns and the Bulgars), a large number of cisterns 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 eant 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 neighbour 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.