Since I think other people will have actual applicable normal ideas, here's a few more abstract jawns
Encirclement and Enlightenment
Local lords throughout the Kingdom were able to raise militia as well as maintain a small standing elite guard identified by their territory's emblem somewhere on their shields, uniforms, and banners. The King, whose forces alone were often not much larger than any singular vassal king's, could be supplemented at any time by the other lord's at his command.
Forces were gathered to repel the raiding and invading Iconan armies. The Army of Many Banners proved quite flexible, as each vassal king's forces provided their own unique attributes to the whole based on their territory. The Army was able to split up to converge on barbarians multiple times in order to repel them, coming from multiple directions and often engaging on very favorable terms. A number of barbarian hordes were encircled and eliminated over the years, and repeated failed attempted border raids and invasions wore on the Iconans.
The Iconans went through periods of recuperation by invading their other neighbors to prepare for another inevitable attempt to get some of Askia's fabled riches. While no attack was a crippling success for the Iconan's, they still managed to take objects and relics of Askian origin back home. This resulted in an underground interest in Askia, it's culture, and the Chiite faith, that eventually laid it's influence onto even the ruling chieftains.
The final raid into Askia went well at first, but eventually the Iconan's were outmaneuvered and cut off. A number of tribal chieftains opted to negotiate with their enemies. During the meeting a few of the Iconan chieftains revealed their Chiite beliefs and offered their lives and lands to Askia, so long as they were allowed to continue overseeing their present lands. This went over well with the Askians present, however the less open Iconan's were furious at the betrayal and the first blows of a long, bloody civil war were struck. The surrounded army collapsed in on itself as news of the surrender spread, and the Askians only had to watch as the barbarians tore into one another. Icona was split between Loyalist and Chiite tribes, and the war would drastically change Askia's neighbor and their future views on the Kingdom.
Askia's loosely structured but well organized armies allowed them the ability to turn away the Iconans at every opportunity, but it was their ability to draw in other tribes - and willingness to do so - that led to Icona's collapse.
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Gold and Gems
The Iconan barbarians (:l) proved to be a strong, militaristic confederacy of tribes capable of staging massive raids across the Askian border. While the assembles Askian armies were capable of holding their own, the mountains proved to be a difficult obstacle, and so conflict between the Iconans and Askians drew out into a stalemate.
The Askians saw an opportunity in the Iconans, and soon negotiations were underway as the Askians attempted diplomacy.
The Iconans raided for riches, and while they also loved a good fight, the continued engagements between forces on both sides was proving itself not worth the blood spilt. To this end, Askia would ensure the Iconan's could maintain themselves by providing some funding for their military endeavors. In return, the Iconans became an attack dog of sorts for the Askians. If Askia was in conflict with another nation, raiding parties of Iconans would be attached to and under the command of Askian forces.
Within the savanna and desert were vast veins of gold, silver, and clusters of gems that made the military funding an easy effort - the Iconan's couldn't be faulted for having attempted to take some of Askia's vast wealth. Over time this meant a larger portion of Iconans spent time in the Askian military in some manner, and relations eventually turned positive.
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Walled Cities
The barbarians of Icona were hardy, large, and bestial. The would fall upon caravans and villages and vanish. War between the kingdom and the barbarians was bloody, but the Iconan's were not a people equipped for tackling Askia's greatest feat of the Classical Era: massive stone walls. Cities were ringed in at least one layer of imposing walls set up with murderholes, towers, heavy iron gates, thick wooden doors, and a garrison of defenders. Larger cities with access to siege weaponry would also use them on the walls to break up larger formations, prevent setup of opposing siege equipment, or sheer intimidation. The Iconans had a difficult time getting over, under, or around the many walls Askia constructed around it's vital operations, and as time went on the Iconan raids became weaker, smaller, and less aggressive. After a particularly light Raiding Season Askia realized it's repeated invader was finally weak enough to strike at in their own homes without much worry.