This is where you find the all-important Tower Key, which unlocks multiple paths and secrets
Head back to the room where you first used the Scorching Iron Scepter and this time take the lift on the right. This goes down to a lower level where you can run across a hallway and into a hall with two flaming statues.
There are a bunch of enemies with exploding barrels here, so get their attention and head back a little to put some distance between you and them. Hit one with a ranged attack and the resulting explosions should be enough to kill them all.
Out of the soot will come a Giant Club Demon. Take him down and another two will emerge, so take you time and fight each one separately if possible. When they’re all dead there are plenty of goodies to sweep up; a Skeptic’s Spice, two Wilted Dusk Herbs, two Titanite Chunks, a Tower Key, five Charcoal Pine Resin, two Twinkling Titanite, and two Human Effigy. Once you have the Tower Key, go here and use it in the areas where you’ve already been through so far.
Go back to the large circular room and you can either head directly to fight the Fume Knight boss or head to another area and take on the optional Smelter Demon boss by going down the Iron Passage.
However, if you want to continue in this area, go back to the room where you used the Scorching Iron Scepter and open the door behind the minotaur head.
Use ranged attacks to kill an enemy at the bottom of the stairs. When you get halfway down there’s another curse being caused by an Ashen Idol below. When the Axe Demon and the invader approach, go back up the stairs in order to fight one at a time. down the left hallway you’ll find a room on the left with three Old Radiant Lifegems and a room on the right with a red invader.
Down the next set of stairs is another Axe Demon and there’s another down the hallway to the right for good measure. Two Possessed Armors are waiting down the hallway to the left with five Large Titanite Shards at the end of the hall. There’s also a door on the left down the left hallway and inside is another Ashen Idol that gives you a Soul of Nadalia, Bride of Ash when you use a Smelter Wedge. Destroy the idol to remove the curse.
Now, down the hallway to the right there’s another room with a chest containing a Fire Snake Pyromancy spell. Once you have the spell, all of the items in this area have been collected so you can go to the main boss, the Iron Passage and the Smelter Demon side boss, or go back for items accessible with the Tower Key and lifts active.