There are a few Dark Age / Medieval states that really cought my fancy, states that today people mostly forgotten but were really fabulously wealthy in their own right, I'll start with one of them here, the Dark age state of Khazaria.
To give a brief intro:
Khazaria was the most dominant Dark Age period state in the Russian Steppes, it emerged after the chaotic mess after the disentergration of the Hunnic Empire. It's core regions were the northern and western coast of the Caspian Sea, espeically in the Volga Delta region.
It is most interesting because it was quiet different from most nomadic confederations in the history of many kind, most of them formed rapidly under the leadership of a powerful ruler, they were maintained mostly through raiding and plundering neighboring civilziations, and as their success grew their size grew as well until they could no longer expand their conquest, then they suddenly implode.
However, Khazaria was not like that, it was like most other nomadic states, a confederation where most of the population were allied tribes who's loyalty hinged almost entirely on the profitiability of the Khazar's alliance, but unlike other nomaid states, it's chief means of profit was not raids and plunder, which was most likely why it also sustained it self far longer than most . (it started around the 7th century and lated till the early 11th C.)
The Khazars' on the other hand, was in fact primarily a merchentile state, it's chief profit was through the maintainese and control of the Northern Trade route, which through its' power became far more accessable (since your not as likely to die to the attack of a million nomads). They also took advantage of the fact that Silk Road was primarily controlled by the Perisans, but amongst their biggest end consumers, the (eastern) Romans, were usually in a hostile situation with them. Thus the Romans had signfiicant incentives to seek an alternate source and route to access the goods of the Silk Road that isn't held by hostile nations.
That route was developed by the Khazars, which created an alterate route where you go north beyond the Greater Persian zone, into the steppes, eventually down the Don River and into the Black Sea and then from there sail to Constantinople. it is a fast route really, as there are far less terrains barriers (in the summers anyway) and far more navigable rivers. the only real problem that kepted it from operating before was the simple fact that it was dominated by Nomads.
The Khazars managed to built a system where the nomads now instead of plundering the travelling merchnts. would instead actively protect and assist their travel, and in exchange the Merchants would obviously pay a toll which the Khazars would eventualy split up amongst all the tribes, it was a simple but effective system that created immense wealth, aided further by the fact that once such route open, a flood of high quality furs could now be shipped to both Persia and Byzantium, which further fueled their vast wealth. it was said that the Khazar Khagans often paraded through the streets of their capital in Atil (near modern day Astrakhan) and just throw silver coins from a basket at the bystanders.
As their wealth grew, so too did their involvement in the politics in the rest of the World, predominatnely the (Eastern) Roman and Persian (later Arab) wars. as you can probably see from the basis of their economy, they were obviously more likely to have sided with their customers (the Byznatiums) than their commercial competitors ( the Persian / Arabs) thus most of the conflict involved Byzantium / Khazar cooperation , the Byzantiums even sent a huge team into the middle of the steppes and erectected a huge fortress for the Khazars as a return favor. the famed fortress of Sarkel which was located around where the Don river flows the closest to the Volga (which should give you a good hint as to how importantly the Khazars view theses river as part of their interest)
The Khazars sent armies into Persia in coordiation with the Byzantiums on several occasion, and later on they fought a series of wars with the Arabs, with one Khazar attack going as far as Mosul (Iraq) and an Arab attack up to the western Caspian coast beyond the Caucasus, however genereally speaking most of these wars didn't do much in the greater picture of things.
The other thing the Khazars are prehaps most interesting was the fact that.....
They were the only state in the entire history of man kind that converted to ....... Judaism
That's right, after the Rise of the Arabs the Khazars apprently sees the need to have a monothesis religion on their side as well, to strenghten their internal unity, but they decided to choose one that was neither Christianity nor Islam...
However, soon after the Converstion the Khazars began to decline, part of it was the convertion to Judaism caused rifts within the allied tribes, some tribes (most notablly, the Magyar tribes) decided to simply pack up and leave (which would become the events that lead to the foundation of Hungary). these events cause the Khazar's confederation to become destablized, but perhaps the more influential event was the arrival of the Viking Age.
While in Western Europe, the Viking rage is mostly remembered for the plunders along the North Sea (particularly the British Isles) and the founding of Normandy, but it's effect was arguablly far larger in Eastern Europe, where Vikings began founding new trade colonies in modern day Russia, the most famous of which was Novgorod being founded by one Rurik, from that point on the Vikings in the East (called the Varagians by the Byzantiums) would become known as the "Rus" the term that would eventually become "Russia"
At the start, the Rus people mostly traded along the upper Volga, bringing furs down stream for Persian Coins from Khazaria back to the North Sea region (which is why digs of the era as far as Norway often finds substaintial amount of Silver Persian Coins). however, they eventually began to expand their influence, partly to get out from under the Khazar yoke, as obviously the Volga trade was almost entirely under their control and thus profits were often limited. The new trade route they opened was the one along the less navigable (but not for the Maritime based Vikings) Dneiper river that also leads into the Black sea.
With the weakening of the Khazars and the departure of a lot of their western Tribes, the Rus moved south to fill he void, here they founded a new city along the Dneiper that would become the center fo their operation and realm for centuries to come.....Kiev.
The Rus moved their center of operation to Kiev in the early parts of the 10th C, most likely under Igor I (who is said to be the son of Rurik). when Igor died he left the throne to his 3 year old son, but his son Sviatoslav I would end up as a huge player and one of the most important figures in Europe at the end of the Dark age period.
Under Sviatoslav I (a really legendary men himself in many rights) the Rus began a series of offensive agains the khazars, and in 967 AD basically destroyed them by burning their capital to the ground, while the Khazars clung on for a few more decades it was never the same again. and finaly in the early11th C it disappeared altogther.