Iron Keep (2)

Trapped Hallways

To continue with the main path, enter the furnace and loot the corpse in the corner to get a Large Titanite Shard x2 and a Petrified Dragon Bone. Climb up the ladder and defeat the knight captain and loot the corpse beside the ladder to get an Old Radiant Lifegem and Green Blossom.

Climb the ladder and defeat the knight captain in the next platform. Next, engage the two shelled knights. (These enemies can be farmed for their Large Titanite Shard drops) One of them will destroy a portion of the bridge which you need to jump across. Cross the bridge and defeat the heavy knight. Another knight will run towards you so take it out as well.

The two heavy, shelled enemies can be killed by pulling the lever. This will lower the platform they're on and kill them easily. The heavy door to the left has a Pharros Contraption behind it. Insert a lockstone to reveal a door. Destroy it and go up the ladder.

You'll find a bonfire but don't drop down there yet; loot the chest on the far side first to get a Thunder Quartz Ring +1. Now drop down and activate the bonfire. Rest if you need to then climb the ladder. You'll find another dwarf here that can make you join the Bell Keeper covenant. Entering the mist will take you to Belfry Sol.

Back in the lava room with the hanging platforms, go upstairs and take the stairs down. This will lead you to a ledge with a corpse and a knight. Further on are treasures on lava rock. Like before, it will be up to you whether to risk your life and limb to loot these.

Be careful not to step on the pressure plates as it will cause a continuous flow of lava on the canals. Loot the corpse to get a Large Soul of a Proud Knight and Radiant Lifegem. Backtrack and step on the suspended platforms. Loot the Large Soul of a Nameless Soldier from corpse hanging on the edge.

There is a ladder at the far end of the platforms and you can reach it easily if you have pulled the lever to kill the two Shelled Knights. Climb it to find jars full of water and a Mimic Chest; kill it for a Lightning Winged Spear and a Dark Armor.

Keep going forward for another chest (real one this time) with Destructive Greatarrow x10 and another ladder. Climb that ladder to find a Pharros' Contraption that only releases a pool of water (Which I believe is useless at this point, even if you're trying to get the Lava items).

Go upstairs to meet three Elite Alonne Knights. Kill them and loot the corpse with a Simpleton's Spice x1 and a chest with a Black Knight Greataxe in the two corners.

Next, carefully drop down the ledges inside the opening and loot the corpses for a Torch and a Cracked Red Eye Orb. When you reach the second to last ledge, your only option is to fall on the border of the lava cauldron under you. Carefully set yourself next to the rightmost chain above the cauldron and slowly walk forward and you should fall safely. Loot the skeleton for a Covetous Gold Serpent Ring, fall down to the last railing and kill the knight. After that, you're exactly on the same room you'd be if you had ignored the ladder and killed the Shell Knight at the end of the hanging platforms' lava room.

(Thanks to Gamefaqs user Manepss for this very detailed tip)

Back in the lava-filled room with hanging platforms, proceed towards the ledge with fire traps. They have intervals so wait for them to briefly stop then quickly get past them. Since enemies can be damaged by those fires as well, you can lure the nearby heavy knight to have the flame kill him instantly. At the end of the path is another corpse carrying a Monastery Charm and Large Soul of a Nameless Soldier.

Go through the safe mist and continue forth. A knight will drop down from behind as you take the stairs down so be careful. Enter the corridor and kill the shelled knight. Pull the switch and get past the blade. There's a shelled knight just around the corner so lure him back so you don't get cornered. The blade will fall down after a few seconds so don't get killed by chance.

Carefully time your movements to avoid the fire traps. Go to the left and loot the corpse nearby to get a Lightning Short Bow +1.

Note: This corpse is a bit risky to loot; since there's a bonfire nearby, you can light up the bonfire first then pull the lever nearby to disable all fire traps then come back for the treasure.

Go up two ladders to reach the bonfire. Upstairs is a lever that will deactivate all fire traps in the area. Go back down the ladder and defeat the last shelled knight. Continue outside and enter the misty doorway for a boss battle.

Boss: Old Iron King

This boss is easy for ranged characters, especially miracle users. There's a safe spot in the side of the building where the boss' fire balls can't reach. Beware of his fire beam as it can penetrate the wall and kill you while you're still recovering back to your feet. All of the boss' attacks can knock you back to the lava, resulting in instant death. Sorcerers who rely on pyromancy will also have to adjust since the boss is obviously resistant against fire attacks.

When you see the boss dive into the lava, quickly change position and run to the other side since he will appear on the same spot where you're standing a few seconds after. When done correctly, he will resurface in the far end of the stage, allowing you to snipe him and protect yourself from his slow but powerful fire projectiles.

Melee characters need to get close to the boss to prompt it to use melee attacks. It will usually be vulnerable after it slams its fist to the ground so use this chance to get up close and land heavy damage to the boss. If the boss uses its fire projectiles and beam, just stay behind cover or evade. If it dives, lure it to the middle of the battleground; that way, you can easily run for cover and attack. Just repeat this pattern to ensure victory.

One does not simply fight the Balrog

After defeating the boss, enter the ruined building and loot the chest to get a Sublime Bone Dust. Go downstairs and light up the primal bonfire. You can also use it to return to Majula.