The dwarves didn't just use the mountain range as a refuge, but also as a weapon of mass destruction.
When the humans invaded the central mountain range, the dwarves retreated and let the humans invade as far as the third mountain, before releasing the weapon they kept hidden.
Without any warning, toxic fumes began to rise through the mountains as the temperature rose by several degrees. Earthquakes started before the mountains erupted. making humans belatedly realize that they were actually inactive volcanoes.
The dwarves redirected the magma flows to power the forges of the capital and major cities, but the dwarves could return the natural flow of magma whenever they wanted.
Needless to say, the expedition was completely annihilated. All ground troops were burned alive or intoxicated by toxic gases. Even mages capable of flight were unable to escape, as the dwarves used spells on a massive scale to separate the element of air and water from the world's energy.
