What is federalism and give an example of how it works?

Federalism is a system of government in which entities such as states or provinces share power with a national government. ... Federalism helps explain why each state has its own constitution and powers such as being able to choose what kind of ballots it uses, even in national elections.

Explanation:

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).