Iron Keep (1)

The knight enemies here are weak against lightning so Clerics or characters with access to Miracles can one-shot the ordinary knights and two-shot knight captains with Lightning Spear for easy farming.

As soon as you enter the area, go downstairs to the left to find the Threshold Bridge bonfire. You may notice an item nearby but you can only get it with the help of several items that can increase your fire defense. There are several items all throughout this level that will require you to do suicide runs to get to those items so I'll just list those items so you'll know if they're worth getting or not. The first one is found under the stairs leading to the first bonfire.

Credits to Gamefaqs user Macroman for the list of items. You can visit the thread to learn the specific method of reaching each item. I deemed them most of them unnecessary so I didn't bother getting them, except for the Human Effigy.

Next, cross the bridge and you'll find another item across the broken rocky path with a couple of burning lava rocks in between.

Continue along the bridge and you'll find some hollowed knights near the entrance. Lure them one by one if you want then enter the door. Take out two more enemies inside. You can't do anything with the lootable corpse immediately so head to the right instead. Continue to the deadend to find a merchant named Magerold. He offers the Incense option which can lower the required attribute to use a spell, Jester and Bandit armor sets, Cursebite Ring, some consumables like a Human Effigy and Fragrant Branch of Yore as well as sorcery and hexes.

Note: The Jester Set has some good effects, like the Jester's Gloves that increases the number of souls acquired. This stacks with the effects of Covetous Silver Serpent Ring you can get from Lost Bastille. You can take advantage of their effects even if you didn't meet the minimum requirements to fully enjoy their stat bonuses.

Once you're done dealing with him, climb the stairs nearby and defeat the lone knight. Loot the corpse to find a Pharros Lockstone. There are also five urns nearby that contains water; if you soak yourself with them, it will give you a little fire protection. The corpse below has an Iron Key so if you have an Small Orange Burr (dropped by the undead in Huntsman Copse), you can use it to boost your fire DEF temporarily. Use it in combination of water then quickly grab the key and roll away before you get toasted.

Continue upstairs and lure the knight from the left side. Defeat it and take care of the archer with the fire bow to the right. Interact with the wall behind him to reveal a secret room with a ballista and a corpse containing x20 Fire Arrows. If you interact with another wall inside the small room nearby, you'll reveal another door that will provide a shortcut near Magerold's shop and the exit. You can now use this as an alternate path to circle around the knights if you want to bypass them.

Head outside and take out the archer on the ledge to your right if you have the means to. Continue forth and take out another knight and another knight captain ahead. Go downstairs and defeat the knight under the walkway. Pull the lever to lower the pathway for you. Do not open the door! Continue to that pathway and defeat the knight. You can also jump to the lower ledge to find a corpse containing Soul of a Proud Knight and Repair Powder.

From that ledge, climb the ladder to reach the upper platform with a chest. Get a Twinkling Titanite and Life Ring +1 from it.

Cross the walkway and jump on the pillar to loot the corpse that has a Large Soul of a Nameless Soldier and Red Leech Troches. After jumping there, you can drop down to the next ledge below and pull the other lever. This will evenly level the hanging walkway so you can have an easier time traversing it later on. Keep in mind that the archer from the distance can still snipe you and the knight captain nearby can follow you and attack you. There's a chest below on the lava rocks. Like the others, making an effort to loot it or not is up to you.

After activating the lever, you can follow the walkway leading to the main valve in the center of the furnace. Operate it to put out the fire which will enable you to enter the furnace and continue to your main objective.

Return to the bonfire if you want then backtrack again. If you're human, there's a summon sign right before you enter the misty door where you can call upon Lucatiel of Mirrah. In the same ledge leading to the misty door, you can turn around and cross the gap to get the Zweinhander sword.

Getting the Dull Ember

From there, you can do a sprint-jump to reach the metal platform. You must aim for the corner of the platform to successfully land on it. This is a risky jump so don't carelessly attempt this if you're carrying a lot of souls you can't afford to lose. After reaching the platform, loot the skeleton by the broken staircase to get a Dull Ember. Go upstairs and climb up the ladder.

You'll find a Pharros Contraption there. The only thing it does is fill the small pool with water, which you can use to drench yourself and attempt to loot the chest on the lava rock beside the furnace. A Lockstone and several deaths is not worth it though so you might as well ignore it.

The Dull Ember is an important key item though since handing this over to McDuff the Blacksmith in Lost Bastille will enable him to enchant your weapons with various elemental and status effects.

The boss behind the misty door is called the Smelter Demon and is entirely optional. You can always come back and fight it later. If you want to try your luck, you can follow this link. The main path is through the giant furnace in the middle of the area, right after turning the large valve in the middle of the furnace.