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With a POD of 1035 and end date of 1800, how could an empire of roughly this size form in Central and Eastern Europe? The borders don't have to match these ones exactly, it's just a general area.
Basically a cavalry based "counter-Roman empire" with the main trade routes being overland, instead of at sea.
Would it be Slavic? What kind of imperial administration would be necessary for the empire to hold together?