Constantine i moved the capital of the roman empire from rome to byzantium because he:

wanted to re-create the glory of Rome in a more geographical strategic location.

the conditions that made the byzantine empire vulnerable to attacks by the muslin turks were the sack of constantinople by western crusaders, the conquests of the sultanate of rum, presence of rival kingdoms and finally the set up of fiefdoms at the empire which were owned by the ghazis (turkish warriors).

increase in population