Building city walls was of fundamental concern to the imperial administration, as is evident from Justinian’s plan to build walls around many cities and fortify areas of major strategic significance.

Together with the political and economic decline that gradually set in from the late 4^th century, natural disasters and epidemics led to significant changes in the form of cities.

Some were abandoned and others shrunk in size and population, while new cities were established at virtually inaccessible sites, their main aim being to provide security to inhabitants. The fundamental objective of fortifications was to ward off attacks by creating barriers, which were set up one after the other so as to guarantee defence. The number and height of walls, the shape and form of towers, the number of gates and their guard systems were all determined by the lie of the land, developments in warfare and prevailing economic and building potential.

As a rule, walls consisted of towers with two or more levels, from which the defenders would attack the enemy, and of high curtain walls thick enough to withstand attacks. Cities on the plains had another lower outer wall punctuated by towers, in front of which was a moat that could be flooded to check the enemy. At the highest point of the settlement lay the acropolis or citadel. With its own wall for defensive autonomy, this was the last refuge for defenders, and housed the seat of church authority and military command. From as early as Justinian times, an intermediary wall divided the city in two, thus providing a further line of defence. The gates opened at sunrise and closed at sunset. They were few in number, since they were established at the most vulnerable points in the fortifications.

As an outstanding set of defences, the walls of Constantinople set the standard for fortifications around Byzantine cities. Indicative of their quality is the fact that the walls withstood siege by the Ottomans for over a thousand years.

Lastly, fortresses and towers were fortifications which, like city-castles, adopted the rationale of successive lines of defence. Their aim was to control strategic positions and passes and defend wider geographical areas. In addition, they provided storage for agricultural produce, accommodation for local rulers and shelter for the population in times of danger.