The study of the symbolism of the Byzantine city becomes more accessible with reference to the crowning building in Byzantium, the church. The combination of a cubic form with a dome, on one hand, a cosmogramme presenting the earth (the base of the complex) and the firmament (the dome); on the other, it materializes in the form of a church. From earliest Christian times on, the church is an image of the world and of the Heavenly Jerusalem.
The main type of the Byzantine church has a cruciform plan. In biblical texts, the earth and the world are presented as square or circular and divided in four parts by a cross, the centre of which indicates the centre of the world. The two plans are closely related. The cupola’s keystone corresponds vertically to the centre of the cruciform plan, its “Omphalos” (-navel), symbolizes the sky, and the square between the four central columns of the church symbolizes the earth. The iconography of the cupola is adapted to its heavenly meaning. The church is also, like the earthly Jerusalem, the centre of the world. The mythical location of Jerusalem in the cosmic centre is reflected in Western medieval cartography, which puts this city in the centre of a circular earth encircled by a cosmic ocean. The construction of a new Jerusalem, and through it that of the heavenly city, has always been the desire of all Christianity. This was the aim of Constantine the Great when he founded Constantinople. The emperor kept both the Roman ritual for the foundation of a city, and the Roman urban and cosmic cross of streets, composed by the decumanus (E-W) and the cardo (N-S). The Christian equivalent of the decumanus was now the “Leoforos Mesi” (Middle Avenue). Constantine’s elliptical forum ( also carrying cosmic symbolism) had at its centre a huge column with a colossal statue of the emperor on top; the base of the column was surrounded by four chapels, together constituting a cross. Both this central element of the forum and that of the later forum of Theodosius II were located on the Leoforos Mesi and are to be identified by the omphalos, the cosmic centre which is also to be found in the church. This symbolic conception by Constantine is corroborated by his interventions in Rome, where he built basilicas linked to the extremities of the urban street cross of the city. With his urban interventions in Constantinople and Rome, Constantine was not simply christianizing the Roman plan, but he very specifically aimed at the realization on earth of the Heavenly Jerusalem. Available data reveals the use of elements of this Christian model in important Byzantine cities, such as Thessaloniki, Monemvasia and Mystras. This settlement model has survived down to our times in the Greek countryside.