Since no objections... I started with the original idea, but may have embellished things a bit. We have... let's see, elements of the ERE, HRE, the Empire from Warhammer more directly, Venice, and Holland.
Humans occupy a large, geographically varied area, touching on the lands of almost every other race in Marduk. They also have a considerable population advantage over essentially everyone else, though their attempts to present a united front are constantly brought down by internal strife. Still, the threat of so many people being united under one banner alarms their more militant (or paranoid) neighbours, especially those who still remember the old elven empire.
The most significant portion of the realm, in terms of both political and economic influence as well as geographic importance, are the lowlands leading to the coast, and the innumerable rivers that cross through them. Many of the lesser rivers merge into one vast swathe passing through the human’s capital, a trade nexus without peer due to the ability to head upstream to virtually anywhere—or downstream to the ocean and the opportunities therein. With substantial work alongside the dwarves, the original river system has been augmented by canals stretching the breadth of the realm, particularly in the capital—to the point that it’s generally more efficient to get around by boat than walking, though the bridges over the central river are a stand-out feature.
Politically, the empire couldn’t get any more disjointed. Though theoretically united after their first emperor broke free of the elves under the shadow of Edel Athran, the divisions of the numerous scattered tribes persist to this day as political divisions fighting in the realm, sometimes in open conflict, but more commonly in political scheming to get their puppet or actual ruler onto the imperial throne itself. More often than not, however, the original bloodline that ruled the capital and its surrounding lands finds itself at the top of the fragmented pile.
Militarily, humanity as a group favour a balanced approach, as well as adopting new ideas and technologies when they’ve been proven, but they aren’t entirely above a ‘drown-it-in-bodies’ solution. Though having the ability to fund a professional army, the already-fractured political landscape means that nobody is willing to trust a single person with that level of force, with only the theoretically-loyal heavy cavalry being standing forces. Instead, each of the various princes maintains their own troop organisation, whether carrying on their traditions under the elves, conscription, a feudal system, or simply funding mercenaries. Despite all forces ultimately having to answer to the Emperor, short of the high elves getting uppity again, they seem disinclined to answer a call to arms en masse.
In order to protect against a coup from the scheming political factions, one of the past emperors arranged a standing contract with the Dark Elves: they would provide an elite regiment with a size corresponding to the nominal force of the Empire, and alongside the pay, trade agreements with the north would be more generous than otherwise allowed. Said regiment has a terrifying reputation as agile berserkers with a preternatural ability to avoid taking an injury, capable of cutting down even a group of knights with nary a casualty. Whether due to valuing the original contract, inability to provide a better deal, or just taking the long view of things, this group has yet to betray anyone yet… though they’re hardly received well in high company, despite their captain holding a rank equal to any of the princes.
Minor point: the coastline mentioned isn’t the North. It’s probably the Western or Eastern, since I imagine that elven lands would be the closest to the actual dark elves. It also explains why they’re politically disconnected, due to distance. And that way it still has a Mediterranean climate.