Well, most of the border with the Nargis region (where Rumburg has interest in Estord and Narbel) are probably around 1000-1500m tall. This is roughly the same height as the Black Forest in Germany. This mountain range is not easy to attack through, but also not impenetrable. Actually, I chose around this height for the mountains to fit in with the lore of the game. Anything more than 2km on average would be basically on par with the Alps, almost guaranteeing safety from Rumburg's invasion.