I guess something like this - add timelines for every known point (city, landmark, ritual site etc.) connect the dots in the same year with one another, then apply some reasonable territory estimate to round out the resulting blob, correct for visible mistakes manually.
Just for fun, how would you envision this process working for empires that weren't based on territorial control of cities and landmarks like the Mongols, or based on territorial control of land at all like the Hansa?
