What was the main role of Knights in the European feudal system in the Middle Ages?

professional soldiers in wartime?

Explanation:

it's a guess

peasants do the dirty work, knights are not land barons and most likely not educated (NGL THAT MAKES SENSE), but serving the country is their duty

process of elimination, it is professional soldiers in wartime

this is not a question its simply asking you to read and write a few sentences and   the question on the bottom its fairly easy, you can do this!

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).