Its workshops are where Paragon Hirol conceived his famous improvements to golem resilience and power, and where Hirol's favorite student developed a method for storing refined lyrium that is still used centuries after its conception. These breakthroughs brought Kal'Hirol great prosperity; its passageways are said to have glittered with gold and silver.
This didn't last long however, as it fell to the darkspawn during or shortly after the First Blight. As the dwarves of Kal'Hirol were evacuating the thaig, five hundred casteless were left behind. Dailan, feeling that the guard would fail to delay the horde enough, proposed to the casteless that they join with him in the suicidal battle, and suggested they prove that they were not worthless as dwarven society claimed. Despite opposition amongst some of the casteless, approximately two hundred volunteered for the duty. It is not known what happened to the others though at least a few attempted to find a route to the surface.
This section contains spoilers for: Dragon Age: Origins - Awakening.
The Warden-Commander successfully ventures into the great thaig in order to eliminate the broodmothers which reside in its lower levels, as well as learn more about the darkspawn civil war. There it is discovered that two hundred casteless dwarves were the last defence of the fortress in order to give enough time for the rest of the population of Kal'Hirol to escape to Orzammar in safety.
In the epilogue, it is mentioned that Kal'Hirol is cleared of darkspawn, and eventually the dwarves of Orzammar attempt to reclaim it. House Helmi ultimately leads a host of dwarves to clear the Deep Roads between Orzammar and Kal'Hirol and, though the losses were great, they are ultimately successful in 9:31 Dragon.