Why would christan areas in a. d 325 be located near large cities and mostly along the coasts and rivers

Explanation:

It was easier to supply and sell goods because they could travel to other cities by boat to trade.

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).