Soulforge Cathedral was the primary base and center of innovation of the Mechanist Order. It was here that Karras masterminded his plan to detonate his Masked Servants, protecting him while all outside organic life was extinguished by the Necrotic Mutox, also known as rust gas.
The cathedral was filled with all manner of Mechanist machines and security devices, including bloodstained Masked Servants armed with the standard Mechanist maces, which may have been made out of Mechanists who were a bit too slow to follow Karras' orders. The cathedral was populated entirely by the Builder's Children, namely Combat Bots, Worker Bots, and Spider Bots.
Garrett and Viktoria infiltrated Soulforge in an attempt to stop Karras from releasing the rust gas into The City. Viktoria drew too much attention to herself in desperation and was forced to sacrifice herself to the plan, and Garrett was eventually successful in penetrating the stronghold and activating the beacon, which drew the masked servants back to Soulforge, and the rust gas was detonated inside the cathedral instead. (T2: "Sabotage at Soulforge")
It is rumoured that by Thief 3, Soulforge was still standing, but as a decaying ruin.