Terran's Ponderous Campaign
Originally (about 1900 years ago; 1494 A.F.) a colony of the expanding Mycaean culture(s). Over the centuries the territory grew, as did the city and Capital of Prospolis.
At this point the Empire is, as an independent, stand-alone Empire, about 630-ish years old (around 524 A.F.).
It’s history is far older though. As part of the expansive Mycaean culture, it’s been “in operation” for at least 1900 years under that influence, and probably even older before the Mycaeans came along.
The isthmus holding the Mycaean States (and probably a bit of Kunac) is the geographical origin of the culture.
Though it started as a colony, eventually it became the “premier” locus for the culture. The “center” shifted, at least in folks’ minds, to Prospolis rather than the States.
So that’s where all the current and recent “greats” (Scholars, poets, and so on.) are from; the Empire.
All of the major east-west trade routes (all the way to Tana ) pass through the Empire. Trade up and down the Chain Lakes is dominated by the Empire, and terminates at Prospolis. All of the primary ports in the eastern Mare Mercuria are controlled by the Empire.