"The shafts ran down by many slopes and spiral stairs to caverns far under; there Saruman had treasuries, store-houses, armouries, smithies, and great furnaces. Iron wheels revolved there endlessly, and hammers thudded. At night plumes of vapour steamed from the vents, lit from beneath with red light, or blue, or venomous green."
—The Two Towers
The Furnace is one of the primary resource buildings of Isengard. The Furnace's efficiency smelting metal for weapons and armor allows it to reduce the cost of unit upgrades from the Armory, but only once it reaches level 3.