Return to the level where you used the Scorching Iron Scepter and use the lift on the right this time. This takes you down to lower levels, where you should run through the room to the passageway on the far side.
There are two flaming statues here, along with a bunch of explosive barrel enemies to either side. If it's not possible to target them from the hall, dash in to get their attention and then dash back out until you can get at least one ranged attack in. That should blow them all up.
Then there's another Giant Club Demon to contend with. There are also another couple hiding in the soot, so be careful where you tread or else you'll need to deal with several at once. Kill them one by one, then loot the soot for a Skeptic's Spice, two Wilted Dusk Herbs, two Titanite Chunks, the Tower Key, five Charcoal Pine Resin, two Twinkling Titanite and two Human Effigy. Cor. With the Tower Key pocketed, you can skip ahead to section 3A of this guide if you want to use it immediately.
In the meantime, go down the hall and into the big round room. If you want to take on the Fume Knight boss immediately, go to section 2B of this guide. Or you could go to the side area and fight the optional Smelter Demon boss, which we cover in section 2C, dealing with the Iron Passage section of the DLC. Or you can continue in this area, in which case head back up to the room where you used the Scorching Iron Scepter, open the door behind the minotaur head and read on.
There's an enemy at the bottom of the stairs, so range him to death. Before proceeding, though, do use an item that reduces or cures curse (Divine Blessing, for instance), because there's an area halfway down filled with the stuff due to an Ashen Idol located further down.
At the foot of the stairway is an Axe Demon and an invader. Move into the passageway at the bottom, then quickly retreat up the stairs and you should be able to kite one and deal with them separately. The left hall leads to a chest containing three Old Radiant Lifegems, while the right has another red invader enemy but no goodies.
Keep on down the stair and there's another Axe Demon, then one more in the hallway on the right. Two Possessed Armors are lurking in the hallway on left, although you will at least get five Large Titanite Shards for your trouble, and there's a door on the left in the left hallway leading to a room with an Ashen Idol, rewarding you with a Soul of Nadalia, Bride of Ash when you disable it. This also removes the local curse effect.
Along the passageway on the right, there's another room, containing a chest with the Fire Snake Pyromancy spell, which is the last of the loot in this area. With that safely stowed, you need to decide what to do next. Follow 2B to go straight to the main boss, 2C for the Iron Passage and Smelter Demon side boss, or 3A for more items accessed via the Tower Key.
Or not to be, depending on how you do against the Fume Knight. Return to the Foyer bonfire in Blume Tower and use the lift that goes down. Take the next lift here to go even lower. As it descends, swivel round and you should see a pair of routes. Run and jump as the lift nears the uppermost and there's a room here with a chest containing a Strength Ring. Ranged attacks let you deal with any enemies visible through the hole here. Fall through and deal with any stragglers - it shouldn't be too difficult at this stage, but be mindful of the Axe Demons with flaming weapons.
When you get to the fourth and fifth floors down, there's a crumbling floor section in the corner. Walking over this sends you tumbling several levels (this also affects enemies). As you descend, you'll locate a Partizan +6 weapon on the sixth floor just below the bottom floor. From here, there are three enemies below: two Axe Demons (one's a flaming axe guy) and one Giant Club Demon. They're all vulnerable to ranged from your safe position, so go with that if you can, and be wary of falling damage as you drop down.
At the bottom, pull the lever near the big door. There are two more Axe Demons and two Possessed Armor guys ahead (one of whom plays possum until you approach). The Ashen Idols below have to be disabled before the Possessed Armors will fall, so run past all four enemies and down the stairway on the opposite side. The Possessed Armors have wide-ranging attacks, so watch out as you pass them.
Prior to the stairs is a path on the right leading to a chest with a Sorcery Clutch Ring (boosting magic attack) in it, but come back for this after the idols and enemies. In the room ahead, ascend the stairs and hop onto the lift to lower it to the level below. Step on it again to send it back up though, since dying doesn't reset lift positions, as you are bound to have learned to your cost or inconvenience at some stage of your Dark Souls adventures.
OK, up the stairs are four Ashen Idols (one on each side). A Giant Club Demon will pop up out of the soot near each one, too, and attack. You need to deal with at least two of the idols before the demons will die.
Ahead should be the fog door that leads to the battle against the Fume Knight, but do try to deal with the Ashen Idols before you go through it, because they also heal the boss when he gets close to them.
Don't miss a single weapon. On the first page of our walkthrough, we have links to individual Crown of the Old Iron King weapons and rings from across the adventure.