Being based so far away from any civilized lands, the city could not be very large, but as you say, economical and political terms, extremely important.
Have you given any thought as to what kind of trade this city deals in? There must be a reason for why the city's is so important, even though trade with nomads and barbarians was extremely profitable in ages past (fur and other un-processed resources). Perhaps the tribes bring wares from an advanced kingdom even further away, such as spices, precious metals or whatnot? If the resource is some kind of drug or narchotics, that has made the upper classes of the civilized continent addicted (with perhaps supernatural side-effects, as well as negative ones), things become very interesting.