Christian churches
During the first three centuries of Christianity, Christians worshipped God in simple buildings, many of which had previously been private homes. Once Christianity became the official state religion via the Edict (Order) of Constantine and Maxentius in 313, churches began to acquire special features and became central to the life of Christians.

The first churches were founded by Constantine the Great on land belonging to the emperor or the state. Over the years, many public buildings and pagan temples abandoned by worshipers on the emperor’s orders were converted into places of Christian worship. This change helped to maintain and preserve them until the present, since if they had been abandoned they would definitely have fallen into ruins.

Churches were usually built at key sites in cities, close to main streets or squares. They were the central points around which neighbourhoods were organized, and at the end of the Byzantine period they even lent their names to the surrounding area, a custom that has continued up to the present day.

Throughout Late Antiquity only the bishop had the right to celebrate Mass in all the churches of the city. Normally the Matins or the first part of the Divine Liturgy was celebrated at a church and the rest of the Mass continued in another one, while during processions the celebrants chanted in the streets. A priest could only say Mass in certain churches if he got special permission from the bishop for particular reasons, as in the case of private churches built by wealthy owners for themselves and members of their families.

The permit was supposed to prevent gatherings of heretics. It is no coincidence that the temples where everyone could attend the mass were called catholic or “universal” churches.
 
Monasteries
Apart from churches, Byzantine cities also had monasteries. Although monasticism represents some people’s desire to devote their lives to God and distance themselves from social life, after the 6th century monasteries began to be founded within the city walls, becoming centres of intellectual activity with significant economic power.

Every male or female monastery was run by an abbot or abbess, in accordance with a rule governing common life within it. It was surrounded on the outside by a high wall (the enclosure wall) and had several buildings, depending on its size, wealth and the services it offered the city. Monastic life was organized around the main church or kyriakon, which was usually surrounded by the remaining buildings. The refectory (dining hall), kitchens, ovens and monks’ cells were among the most essential of these. Equally important were the storerooms, the guest areas and the baths. Monasteries that were pilgrimage centres also had galleries and chapels to accommodate anyone who stayed in the hope of being healed by the miraculous saint that the particular monastery was dedicated to. Other buildings served as hospitals for both monks and lay patients, workhouses, orphanages and schools. In some cases monasteries were built at important points in the city in order to have better control over them; for example, Vlatadon Monastery in Thessaloniki was built next to a large underground water tank, where one of the supply channels that brought water from Mount Hortiatis ended.

In other cases, monasteries situated at the edges of the city next to the walls may have strengthened defences, as in the case of Vrontochios and Perivleptos in Mystras. Lastly, many monasteries had good libraries, while some were important centres for producing and copying manuscripts books, such as Stoudios Monastery in Constantinople.