Natural Divides

While some regions of Domhan are more mountainous than others, that are only a handful of mountain ranges that serve as natural divides.
First is the mountain range to the north-west of the Junction, called the Guardians, separating the rest of the Junction from the The Disjointed. Though the creatures beyond rarely dare to make it through the mountains, some of them still do - and must be swiftly put down.
Then, there is the range that separates the Junction and the Holy Dragon Empire. Derisively called the Dragon-Catchers in the Junction, and Nyoka's Teeth by the Holy Dragon Empire, these mountains are more of a small deterrent than anything else. After all, dragons are able to fly, and therefore would be able to ignore the strategic disadvantage imposed by these mountains. Luckily, their eternal civil war prevents them from exploiting this… for the time being.
Finally, there is the mountain range that separates Welmenni and Vengeance. Both sides call this range the Rampart. Out of all the natural divides, this one sees the most action, as the Children of Domhan are permanently trying to break through the defenses built by Rafinova; to very little success, as the latter's technological advancements have allowed for anti-air and anti-sea defenses to be installed nearby, making Rastlin's usual brute-force approach quite ineffective.
There are also two divides that, while not really separating a country from another, can still pose a challenge to armies that would seek to move beyond them. First is the river (and its deltas) that split the Junction in two. The main branch is called the Shield.
The other is the mountain range south of the Free Domhan border, called the Crossway. This range was recently erected by the twin gods, Trokair and Zakon. Though it may look innocuous, it is a defensive measure in case the people of the Domhan Reformation Army manage to push their border further south. They would then be forced to make a choice: to take the long but safer way by going east, or to take the shorter yet more perilous road by going west. Incidentally, it also poses a logistical challenge for Vengeance and Rastlin, even though that was not their original goal.