The Korean’s famed for archery therefore the dwarves would need to counter this. One way is heavy plate armour. Another would be wall shields (stretched hexagonal). They would need missile weapons too. To distinguish them from other dwarves maybe no set firearms but hand held miniature rockets launched along a wooden track like a cross bow without the bow. You could have a series of these ranging all the way up to large rocket batteries. Crossbows too could be used but stylistically with Korean heavily recurved bows. Maybe phalanxes of spear wielding heavy infantry protecting rocket batteries and lines of heavy armed half held rocketeers. You could convert crossbowmen with removing the bows adding a rocket and a backplate behind it. These guys would need beard plates and face armour. Possibly the stylised face plate of late shogun era. They would need protection from the air... what about leaders on winged Kirin? Magical horned winged goats of the Mountains.
The independence of these holds could be maintained similar to the Swiss of the 16th century. Too much effort to conquer and happy to be hired as mercenaries by the empire to the west. Whilst acting as a barrier to that empires expansion into the human lands of the peninsula to the east