the Imperium is designed to be "the Ancient Empire whos "golden age" all others aspire to" inserted into a game world as history.

As a template, you can think of them as a Roman Analog society. When confronted by a needed image or area, that is not covered by write up, assume it is the least corrupt Roman option.

If you want a "quick" read, Dokoren - The People of Legacy - A new take on Dwarves are descendents of the Imperials. And look at the the freetext Imperium http://www.strolen.com/view.php?node=free_search&free=Imperium

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Summation
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Long ago, There was The Imperium. This is the great Roman Empire Analog that had great powers of old (now lost) and a client-patron system that will evolve into the psuedo medival system found in most fantasy games. Something bad happened. The Imperium fell due to missing magics, resources not being transfered, and lack of communication. The reminants of the Imperium are the foundation in which the "current world" is now built.

Imperial culture is one built of formal rules of law and duty. Everyone knows their place and what is expected of them. You are a patron of your lessers and a client of one of your betters. This interplay of obligations is the center of Imperial Culture. They are belivers in ORDER, DUTY, PRIDE in one's self, ones patron, one's people, and one's city, and the skills to back up that pride. While hubris is frowned upon, proper belief is crucial to the Imperial Mentality.

Imperial Culture had a rank system. Nobles were those of Power and Wealth. Now Imperial Magics were not the combat magics and odd spells that have survived the fall of the Imperium. They are practical magics used for building, communicating, summoning elementals for use, travel, and the occasional duel and crowd control. The Nobles had access to these powers and used for them for the good of the Community in general and their clients in specific. The clients then provided the nobles with things they needed.

Please note there were Slaves in this culture. Slaves were "leased" from The Imperium. Most slaves were paying off a personal or family debt. In fact foreign soldiers captured by the Imperium had to pay off their capture as being slaves for a time.

The Imperium covered the entire world and some say beyond. They used a system of gates, The Imperial Gate System - Twilight Tunnels to move goods and people across the entire world.

While they used magic, they had a strong technological base. The Imperial Nobles encouraged small scale technology as it freed the Nobles from doing lesser tasks. Therefore they have plumbing, air conditioning, gas pipes, stronger alloys, and optics. They had steam power for a variety of purposes (with engines empowered by the fire magic enchantments of the Nobles).

Their traditions of Duty and Pride in ones self, society, and work, makes for an incredible worth ethic. Craftsmen here make things of intricate detailwork and artistic value (as long as the artistic elements do not compromise efficiency).

The Imperial Lines were never tall people, though many tall people were conqured and added to the Imperium.