From the first bonfire, head over the bridge and you'll face two Alonne Knights. These guys are fast and do bleed damage, even when guarded. They deal heavy damage with their katanas, but are pretty easy to kill, just keep your guard up and let them attack first. Lure the first one out either with ranged attacks or simply by proceeding slowly. Head inside and turn immediately right, ignoring the two Alonne Knights in this room for now.
Merchant Magerold of Lanafir is at the end of this passage, whom mainly sells spells for mage characters. Backtrack down the hall and head up the stairs, where there's another Alonne Knight up the first flight. Turn left and grab the Pharros Lockstone from the body over-hanging the ledge, then head back past the stairs, pressing the action button along the wall to reveal a hidden room.
Keep heading through and you'll reach a ballista, there's another illusory wall where it's facing, hit the action button to reveal it. There's also fire arrows in here too. Once the wall is moved, there's an archer just on the other side, you can use the ballista, but it only does 170 damage, so backstabbing may be more effective. Once the archer's down, there's two more knights in this room to kill.
Turn left if you want to kill the other two knights previously ignored. The large stone ornament spitting fire is protecting an Iron Key, but running through and grabbing it will most likely mean dying.
Head back up the stairs and outside, this area is filled with archers, so it's important to stay behind the pillars to avoid their fire until you kill them. Head along the left and lure the first Alonne Knight back behind the pillars to avoid the archers. Turn right, head down the stairs and kill the second Alonne Knight, constantly moving to dodge the arrows. Don't open the door! This room is filled with fire that is an insta-kill, don't open it yet as you can turn this fire off in a moment.
Pull the lever then head back up the stairs and sprint to the far end below the platform with the archer. Drop down the gap with the ladder and grab the soul at the end of the platform, then climb all the way up the ladder and kill the archer, grabbing the Life Ring+1 and Twinkling Titanite from the chest before dropping down to the middle ledge again.
Head back along the path and turn left across the bridge lowered by pulling the lever. Lead the knight back behind the pillars before killing him, you're a sitting duck on that bridge with two archers taking shots at you. Run across the bridge, turn left and kill one of the pesky snipers found by the mist, but don't go through yet, that leads to the mini-boss, the Smouldering Demon.
Head in the opposite direction, jumping the gap and open the chest to get the Zweihander. You can jump the gap here into the door to the right, just be careful not to over-shoot and land in the lava - you can be extra safe by jumping into the doorway. Grab the dull ember from the fallen body on the broken stairs, which you can give to McDuff in The Lost Bastille to allow him to upgrade your weapons. Head up the stairs and kill the archer. Don't bother using the Pharros' Lockstone here, it just gives you a health pool.
Drop back down onto the path and turn left back onto the bridge, jump to the door with the rotating handle and use it to turn the fire off inside the chamber. Head inside by following the ledge to the right, where you can grab a Large Titanite Shard and petrified Dragon Bone from the dead body. Ignore the ladder for now, as this leads to the main boss of the area, but you want to kill the mini-boss to gain access to the second bonfire.
Exit the chamber back and head back over the bridge towards the mist. If you're a human, and have engaged with Lucatiel at each point thus far, you can summon her before taking on the Smouldering Demon. She's a big help as she takes its attention, giving you time to heal and deal some damage of your own. She doesn't deal much damage to the Demon, but can at the very least distract it.
Proceed through the mist and prepare for a very tough encounter.
How to beat the Smouldering Demon
Be mindful of your health bar as it will decay pretty quickly if you're anywhere near the demon in the second half of the fight (once his chest emits large flames). Equip any armour that has high fire resistance (if you have none, don't panic, just attack and get away quickly).
The Demon does large sweeping attacks which are hard to dodge, as well as a jumping plunge attack which, if it hits, it's pretty much game over. The good thing is this plunge has little range, and the last hit of many of his moves has a long recovery time. Be careful of rushing in and attacking while his sword is stuck in the ground, the Demon can emit a heat blast that does a ton of damage.
While the Smouldering Demon is summon fire from within itself, its damage resistance increases. You can rush in and attack without getting hit, but you won't be as effective.
Let Lucatiel get its attention before running in and swiping the back of its legs. Mages can stay back and take shots from distance to avoid the radial damage, too. This fight's tough, but once you learn his attack pattern, he'll soon fall.
After defeating the Smouldering Demon, you can buy its armour from Maughlin the merchant back in Majula, as long as you have spent over a certain amount of souls buying things from him. You'll know you have done so as he'll be in a slouched back position in the room, and will have more things to sell.
Head up the stairs and open the chest to grab the Porcine Shield and Flame Butterfly. The Porcine Shield isn't very effective, but causes pigs to flee at the sight of it. There's also a bonfire just ahead.
From the second bonfire, open the door ahead, this will lead to the path where the ladder took you from the fire chamber. Kill the lightning archer just ahead. There are two turtle guys through the doorway, one will smash the bridge, creating a gap, which is a good thing as it allows you to take them out one at a time. Kill the first turtle on the bridge, then jump the gap and kill the other one, being mindful of the archers in this new area.
Pull the lever to lower the platform ahead, killing the two turtles in wait by dropping them into the lava. You can open the door to the left, use a Pharros' Lockstone to reveal a hidden path leading to Belfry Sol. Don't head here yet, instead, exit the room and continue up the stairs to the left, heading down the stairs on the other side. There's an archer at the end of this pathway. If you step on the panels on the floor, lava will pour into the channels, stepping on it is an instant death. Kill the archer and grab the item.
Head back onto the platform suspended on chains, killing all the archers up here, there's a soul on the top-left quadrant and then head across to the ledge on the far side. Lead the final turtle into the fire being spat out by the huge ornament, it'll die pretty quickly. Grab the soul and Monastery Charm before proceeding through the mist.
There's an Alonne Knight to the left as you walk through the corridor, kill him before going down the stairs. There's two more turtles through this narrow passage, separated by a guillotine. The best way to deal with these is by getting close to make them attack, but not running too far away to make them chase you with the mace held high, as they won't stop until they hit you. Unless you're a light-armour/mage build, it's hard to escape these attacks.
Once these two are down, you'll come to a room surrounded by fire statues. Ignore these for now and head left up the ladder to get to the next bonfire. From the bonfire, head upstairs and pull the lever to turn off the flames from the statues. Pop back down the ladder and grab the Lightning Short Bow+1. Head down the stairs and lead the turtle back into this main room. You don't have to fight him once he's in the room with the statues; you can opt to sprint straight past out onto the roof, as the Old Iron King boss is just ahead through the mist. If you do choose to engage, it's probably a good idea to turn the flames back on and lead him towards them for a quick kill. Once it's dead, head to the Old Iron King.
How to beat the Old Iron King
The Old Iron King is a really tough fight thanks to the lack of space to escape its attacks and the fact its most devastating move can pass through the wall which acts as your only semblance of cover.
If you have any armour with high fire resistance, don that, otherwise try and go into the fight relatively light so your roll isn't encumbered. Be mindful of the small hole by the entrance to the fight - as you'll be locking on to the Old Iron King, it's very easy to fall in and die.
There are key attacks to watch out for: firstly, when the Iron King raises his right hand, and it's on fire, wait a moment before running at an angle away. The King fires a red laser beam that will take most of your health, if not kill you. You can run behind the wall to make an angle to dodge it, but know that the laser will pass through the building, one of Dark Souls 2's many 'quirks'.
The second attack to take note of comes when he's nearly dead: he'll raise both hands above his head then slam them to the floor, causing a flame shockwave. Again, move behind the building when you see this.
When the King spits out the fireballs, hide behind the building when his head is high. When it's low, stand next to his cheek and attack. Don't get too close, as getting stuck behind his horns will knock you into the lava, another amazing 'quirk'.
He takes a surprising amount of damage from melee, the trouble is he dishes out a ton and offers few chances to be hit. Be patient and he'll be down within a handful of tries. The bonfire beyond the King will return you to Majula.