The Great Foundry is Sigil's biggest and most industrious supplier of steel and steel products. A massive structure, easily ten times the size of the largest Prime Material foundry, it produces nearly everything made of steel that the city uses. Although it has the market cornered on horseshoes, nails, and cooking pots, it is not the biggest producer of weapons in Sigil (as that title belongs to the Armory run by the Doomguard).

The Foundry is run by the Believers of the Source, one of Sigil's Factions. They believe the work of turning raw iron ore into worked steel is a good metaphor for their own philosophy of souls being purified and perfected through successive reincarnations towards the ultimate goal of Godhood.

Although the Foundry produces and sells its steelwork to all of Sigil, the forges and buildings inside the Foundry walls are usually restricted to Faction members only, and their friends by invitation.