In my fantasy world (that's... a pretty generous term for doodles on the back of notebooks), the focus continent of Vade has a massive mountain range in the middle of it, called the Jizhu (Chinese for spine), like the Ural mountains dividing European Russia from Asian Russia. Unlike the Urals, however, it's the product of 2 tectonic plates slamming into each other, like the Himalayas. But I digress. The massive mountain range basically divides the continent (about Europe-sized) in half, and on one side the dominant power is the Deltaric Confederation, a loose confederation (duh) of various mountain and forest tribes--namely, the mountain herders of the Tar, who live on the foothills and meadows of the Jizhu, and the forest-dwelling Delan, who are much more widespread and agricultural, living throughout much of the eastern half of Vade in a broad strip from the North Coast to the south. They have since settled down from their nomadic origins and build some very large cities.

On the other side is the Katian Empire, basically an aggressive and expansionist empire founded Kadt the Conqueror, a general in the armies of one of the individual countries that lived on the western half of the continent who seized power in a coup and went on a campaign of military conquest, expanding his empire from the deserts that his home was in to much of the western half of the continent.

There are essentially 2 points on Vade where large armies can pass through the Jizhu mountains. The first is the Andle, or 安德, a very high and very cold mountain pass that goes through some of the deepest parts of the Jizhu. This means that while it is theoretically possible for an army to march through, no army has tried and succeeded. It isn't helped by the fact that there also happen to be a string of fortress cities at the mouth of the pass, too. The second is the foothills of the Jizhu at the southern coast of Vade. This is where the majority of Katian attempts at forcing their way into the eastern part of the continent have gone through. The original Kadt the Conqueror died here, when he led his legions into a Deltaric trap. The 1st, 2nd, and 12th legions, 30,000 footsoldiers, along with the 5,000 light cavalry of the Horse Legion, marched into a fortress line manned by the Confederation's Corps of the South, 50,000 local conscripts, before being blown up by a series of WW1-esque mines packed with explosives. Well, that basically kept the Katians away for about 20 years. Since then it's been a back-and forth war across those foothills.