The fortress originally, like many fortresses and towns, relied on charcoal and coke to fuel a system of smelters and a metalworking station built into the soil layers of the fortress grounds. They were spacious, all things considered, and for the early fortress more than covered its metalworking needs. However, as time passed and the fortress grew it was known that the fortress would need something more permanent: The Magmaworks.
The magmaworks were the deepest point of the fortress, being right above Everoc's molten core, until the construction of The Iron Palace. It is described as surprisingly cool though still largely miserable to be in for more than a few minutes. A large portion of Spearbreakers' population moonlighted as magma smelters.
One of the side effects during exploratory mining searching for veins of gold and iron around the magmaworks was the discovery of Adamantine. This, coupled with the impressive mineral wealth already being exploited (in the form of massive magnetite deposits and several lengthy gold veins,) would arm the fortress military and make the fortress one of the most profitable in the entire nation, even though it typically never operates at full capacity.
There are no permanent residents of the forges besides skulkers, at least that are known to the dwarves of Spearbreakers. Draconik Sankis, however, apparently has a set of laboratories off to one side. The entrance is concealed, and he, Vanya Carena, Wari, and Mr Frog are the only ones who know how to enter.