History questions

1) How did Diocletian and Constantine transform the Roman Empire? What were some of the enduring effects of their reforms? Which ones had little effect?
2) Was the Islamic Empire of the Abbasid era a revised version of the Roman Empire? Why or why not?
3) How did Charlemagne embody a mixture of classical. Germanic. and Christian cultural elements? How did elements of these various cultures help him rule successfully?
4) What impact did the Crusades have on the development of European civilization and Islamic civilization?