The dwarves actually had 10 Sky Citadels, city-castles built inside mountains that would hold the darklands entrances.
Upon their arrival, the dwarves discovered that their migration had unleashed a deluge of orcs upon the surface, and that their enemies had easily overpowered the few peoples who had survived Earthfall and the subsequent Age of Darkness. In order to better defend themselves, and to commemorate the successful completion of the Quest for Sky, they built 10 massive city-fortresses known as Sky Citadels.
Highhelm is but one of those citadels, located at the Five-Kings Mountains.
When they pushed the orcs to the surface, those orcs were scattered around the region surrouding those citadels.
Koldukar was a sky citadel that was built around the Hold of Belkzen region, and was completely destroyed and taken by the orcs that were around that region.
The general accepted history is that the dwarves forced the orcs out of the Darklands, and once they saw the surface of this new world, they decided it was a much better place than the Darklands, so they called all clans to the surface and began their sacking on the nearby regions (a period on their history known literally as The Sacking).
The few remaining orcs in there were enslaved by the other native races. From Hold of Belkzen campaign setting (page 3):
The orcs found that the surface of Golarion was a wasteland—even centuries after the cataclysmic events of Earthfall—with widely dispersed humanoid populations clinging together for protection. These communities offered little resistance against a violent race hardened by decades of warfare, and quickly succumbed to the might of the new arrivals. The orcs’ sensitivity to light wasn’t an issue then, as Golarion’s sun had been blocked by dense cloud clover for centuries. As word spread among the orcs still inhabiting the Darklands that a much safer and easier life of plunder could be had above, they largely abandoned their ancient homes and joined their brethren on the surface. This period, known among orcs as the Sacking, marked their first golden age—a fabled time of dominance over all the other races of the Inner Sea region.
And this is after the sun came back and saved the dwarves against the orc invasion on –4294 AR:
While a few orc tribes retreated back into the Darklands, the rest slowly and painfully adapted to the new conditions on the surface. They built new settlements in surface caves, at the bottom of dark canyons, and in deep forests, where they adopted a nocturnal lifestyle to accommodate their debilitating handicap.