Wikipedia has a list of the largest empires the world has ever seen. I imagine these were often a game of thrones given the number of ethnicities that must have been covered. Borders and Sovereignty has always been a fluid idea and the people must have got somewhat bemused by the changes. Those who used to be in charge but then weren't, must have also struggled. Greatness is shared. All ideas worth aspiring to are impossible to hoard. The end of the Roman Empire led to the Dark Ages (6-14th). The Islamic Golden Age (C8-13th) was brought to an end around about the sack of Baghdad by the Mongol invasions in 1258. The Renaissance then brought a centre of open learning back to Europe. There were pockets of curiosity and the 'dark age' term is misleading. It is hard to hold curiosity down and our best ideas tend to wander.
British Empire (peaked in 1922)
Mongol Empire 1206-1368
Russian Empire (1721-1917)
Spanish Empire in 1790
Umayyad Caliphate (661-750) at greatest extent