What is referred to by 'constantine xi'?

Based on the clue 'constantine i tried to stop greece from declaring ...', what is the answer? What is associated with: constantine is son george ii fled this country during wwii he returned to rule in 1946? Based on the clue 'constantine the great was the first roman ruler to...', what is the answer? What is associated with: constantine xi the last ruler of this empire died fighting the turks in 1453? Based on the clue 'constantines mother st helena is most famous for s...', what is the answer? Based on the clue 'constantinople gained strategic importance due to ...', what is the answer? What is associated with: constantinople officially became this city in 1930? What is associated with: constantinople was the meeting point in 1097 when 4 european armies launched the first of thes…