Officially, this is the same state founded by the First Emperor in 1 IY. In practice, the Empire has risen and fallen and shifted its governance in a variety of ways. Imperial governance has shifted quite a bit over the millenia.
In its current incarnation, the Empire is a hereditary monarchy with a two bodied legislature. The Acropolis is home to a small body of hereditary nobility. Most titles do not convey membership in the Acropolis. Every province elects two members of the House of Towns, every chartered city elects two people, and every great city elects six. In some ages, these houses have basically taken over; in the current age, the Emperor has enough money that he has the upper hand, though sufficiently widespread public outcry can force him to back down. Elections to the lower house are by the nobility in the provincial seats and by those with enough property in the towns. Most people can't vote and express their political opinions by periodically exploding in riots.
Every province has a Graf who oversees it, typically a major noble who is based in another province in terms of his lands. Provinces are divided into Districts, overseen by Barons. Most Barons are also from a different district than the one they govern.
Chartered Cities govern themselves through an elected mayor, though usually only the wealthy get to vote. Voting privileges can be weird, though, as one form of Imperial reward for service is the right to vote somewhere, usually in a hereditary way. This is why there is a group of 983 halflings in Horizon who get to vote for Mayor.
The Imperial Military is organized into 32 legions, I - XXXIII (There is no XIII Legion; it was devoured by the Stone Thief and ever since, any attempt to create a XIII legion ended in disaster). Each legion is 10,000 strong, made up of 10 1000 man Millenia, which are made up of 10 100 man Centuries, which are made up of 10 10 man Maniples. A typical legion incorporates 6,000 Infantry, 3,000 Skirmishers and Archers, and 1,000 cavalry. There are also Imperial Magical and Divine Units, small groups of wizards/sorcerors and priests/paladins who accompany each legion. Special forces assembled for a specific mission are known as 'cohorts' and may be just about anything.
In addition to the Legions, a force of dragons is part of the Imperial Miliary, and a handful of giant golem warriors piloted by humans as well; the most famous is the Emperor's own Giant Armor, the Golden Carapace. It is rarely deployed more than 2-3 times an age, typically as a last resort.