What function do the majority of the world's 10 most populated cities serve?

the founder of the byzantine empire and its first emperor, constantine the great, moved the capital of the roman empire to the city of byzantium in 330 ce, and renamed it constantinople.

the byzantine empire (the eastern roman empire) was distinct from the western roman empire in several ways; most importantly, the byzantines were christians and spoke greek instead of latin.

constantine the great also legalized christianity, which had previously been persecuted in the roman empire. christianity would become a major element of byzantine culture.

constantinople became the largest city in the empire and a major commercial center, while the western roman empire fell in 476 ce.

as emperor, constantine enacted many administrative, financial, social, and military reforms to strengthen the empire. the government was restructured and civil and military authority separated. a new gold coin, the solidus, was introduced to combat inflation. it would become the standard for byzantine and european currencies for more than a thousand years. as the first roman emperor to claim conversion to christianity, constantine played an influential role in the development of christianity as the religion of the empire. in military matters, the roman army was reorganized to consist of mobile field units and garrison soldiers capable of countering internal threats and barbarian invasions. constantine pursued successful campaigns against the tribes on the roman frontiers—the franks, the alamanni, the goths, and the sarmatians, and even resettled territories abandoned by his predecessors during the turmoil of the previous century.