Fortifications around cities
The walls were very important to cities in Greco-Roman antiquity, so fortifications were a priority for emperors. According to Procopius ’ famous work On Buildings, Justinian did more than any other previous emperor to organize the fortification of cities: he spent money to repair and complete the walls of many ancient cities and fortified important places, all of which created a network that protected the roads that travelers, goods and the army passed along.

Although these efforts helped cities in the empire, the difficult conditions that gradually came about after the end of the 4th century (economic problems, enemy attacks, natural disasters) led to changes. In fact, after the middle of the 6^th century cities lost some of their populations and became smaller, while others were completely abandoned or moved to safer places. The sites for new cities were chosen in naturally fortified areas, such as hills with steep sides, places far away from the sea, or peninsulas connected to the main land by a narrow strip of land, so as to give protection against enemy attacks from land and sea. Another very important condition when choosing a site was that there had to be a good supply of drinking water, even when the city was under siege. Land for growing crops, quarries for stone and forests for wood were vital to the survival of people in a town.

The walls – castle building
Although fortification building is an old art, in Byzantium we find new features. The aim of these was to drive back any attack by creating large barriers built one after the other. The number and height of walls, the shape and form of towers, the number of gates and their guard systems all depended on the lie of the land, developments in warfare and current economic and building potential.

The walls were almost always made up of towers between straight sections called curtain walls. The towers were round, triangular, rectangular or polygonal and had two or more floors. At the top they had a row of crenellations (tooth-like battlements) and holes called machicolations for the defenders to pour boiling water, oil or lead on the attackers. The curtain walls were high and thick enough to withstand attacks by siege engines. At the top they had crenellations in front of a corridor (rampart-walk) connecting the towers, where soldiers could move. Behind the walls were stairs leading up to the battlements, with stores for military supplies and guard rooms at the bottom. Cities built on plains had another lower wall with towers (an enceinte) in front of the main fortifications. In front of this was a moat filled with water, which was the first obstacle the enemy came to.

A special wall inside the city offered additional protection at the highest point, called the acropolis. This was the last place for the defenders to hold out; it was the base for the rulers, as well as for the head of both the church and the army. The towers on the acropolis acted as observation points, from which the guards scouted the surrounding area. In some cases, apart from the city wall and the acropolis wall there was a third intermediate wall, dividing the city into two parts, the upper town and the lower town: if the city wall was the first line of defence for the residents and defenders in times of danger, this intermediate wall created a second line of defence, while the acropolis wall served as the third and last line. As in the case of Mystras, the intermediate wall greatly affected urban planning, dividing the city into sections that only communicated with each other through gates and checkpoints.

All communications between the city and the outside world went through the gates. Because they were easy to attack there were only a few of them, where the main city streets ended.

The gates opened at sunrise and closed at sunset. On the outskirts of the cities, outside the walls, there were often hostels with stables and stores for goods, bath houses, and inns for travellers to stay at. Inside the walls there were only a few main streets – the rest were narrow alleyways between houses built right next to each other. Around churches there were usually free spaces used as squares.

The walls of Constantinople are an excellent example of defensive work – a massive project begun by Theodosius II in 412-413 to replace the walls built by Constantine the Great , which were weak, unstable and no longer able to meet the needs of a fast-growing capital city. The new wall also had fortifications on the shore side, along the coastline, and large outdoor water tanks like the Asparos tank and the St. Mokios tank that could supply people in the city with water for as long as any possible siege could last. The fortifications consisted of the moat, the enceinte and the main wall, which was 12 metres high. The new walls of Constantinople took in an area of about 1,400 hectares and were more than 60 kilometres long. Over the course of centuries they were repaired again and again at various points, but lasted for more than one thousand years before being knocked down by the first cannonballs ever used, fired by the Ottomans in 1453.
 
Fortresses and towers
Fortresses and towers were fortifications based on the idea of successive lines of defence. They were situated at strategically important sites so as to control and protect them. At the same time, they served as the seat of the local ruler, as a storage area for crops from the surrounding areas and even as a safe refuge for residents in time of danger. They played a significant part in defending the areas round about, either by facing the enemy or by giving local cities and monasteries advance warning of the danger nearby.