Praise the sun. Our Dark Souls Walkthrough provides tips for surviving the adventure, a guide to the entire world, and hints for slaying bosses.
When you defeat Quelaag and ring the second bell, you'll be treated to a scene of a giant raising a portcullis. You can find the freshly-raised portcullis at the end of the forest path that leads out of the room with the bonfire, upstairs from the blacksmith in Undead Parish.
As you enter the area, keep your shield raised and start forward through the room. Ahead of you, there's an arrow trap, so keep your shield up to deflect its three shots. There are pillars lining either side of the wide center path, and a staircase ahead of you. Positioned at the top of the staircase, behind pillars, are two of the area's guards. They are difficult to see, but you're bound to notice them when they rush you with their hooked swords. They resemble men with snake heads, so we'll call them snake knights. If you're lucky, you can use pyromancy to hurl a Great Chaos Fireball at them and perhaps take them both out at once. If you don't have that ability or are weaker, you'll have a longer fight of it but can still rely on fire. When the monsters are about to attack, they lean back before delivering a stamina-crushing blow, so you'll have time to roll around and possibly backstab them. Note that very rarely, these fellows will drop the Man-serpent Greatsword.
Investigate the area at the top of the stairs to find a corpse along the right side. You can check it out to obtain the Soul of a Brave Warrior. Then you should keep climbing the stairs through another archway.
In the next chamber, you'll notice some swinging pendulums ahead of you. They're sharp and dangerous. Predictably, they swing back and forth across a stone trail that you'll have to follow across a chasm. On the far side of that stone bridge, there's another of the snake creatures. On a ledge above, there's yet another one. The one on the high road will hurl lightning at you as you proceed, so you need to keep your shield raised and you need to move quickly forward while avoiding the swinging blades. Get within range of the snake knight on the path and take him out with fire, then rush forward and hide behind the stone wall for protection.
Here, you can head left to find some stairs. When you climb those, keep an eye to the left where the snake knight is positioned on higher ground. It will likely continue to throw lightning shots your way. Climb to the top of the stairs and look around the wall. There's a path that leads forward to where the monster is waiting, but you'll have to pass through some more of the huge swinging blades. Keep your shield up and run past the enemy, through the opening beyond and then quickly veer left. If you did it right, a trap in that room will fire several arrows at your foe as he turns around to chase after you. He'll be killed.
In the room you just reached, there's a treasure chest. When the path is clear (after the trap has sprung), open it to find two of the Large Titanite Shard.
Head right from the chest and up a short set of stairs to find yourself on another walkway. This one proceeds forward toward an opening, and ahead of that opening there are some stairs that descend to the lower left before wrapping around to the right where a snake knight is waiting. From your vantage point, you can toss firebombs or other fire-element attacks at the knight and kill it before it can even get close to you. Then proceed toward the opening you saw. You'll likely attract the attention of another snake knight who is patrolling just beyond that opening. Kill him and start for the opening, but be careful not to pass entirely through it.
Just beyond the opening, there's a hard-packed dirt path. That path is a trough. If you look to the right, there's a drop-off. To the left, the trough continues for some length to the base of a flight of stairs. A huge boulder regularly drops, rolls down the stairs and then along the length of the trough. Wait for one to pass you, then immediately run forward and up the stairs to the first landing. Then head quickly left to find a doorway shrouded in mist. From there, you can wait for another boulder to drop, then run partway up the stairs and drop off onto the right side to find a weapon known as the Shotel. You can also proceed uphill to find an enemy that you can defeat to get your chance at a treasure chest that holds the Ring of Steel Protection (quite useful). Or you can just proceed through the mist-shrouded door directly.
Once you pass through that opening, you'll find a small chamber with statues along the sides. Ahead, there's an opening. If you try to start through it, you'll find another of the snake knights. Lure him to you, then take him out comfortably. Now pass along that corridor and keep to the side to avoid stepping on a floor plate that will spring a trap behind you and cause you to get hit by several darts.
Head through the next room and start up some stairs, but stop short of passing through the opening. Watch the area ahead of you for a moment and you'll see a huge boulder go rolling past. This is part of the same trough that you negotiated earlier. You'll need to wait for a boulder to roll past you (don't step out too far or you'll be clipped by the boulder; it comes closer to the wall than you might think), then you'll need to immediately run uphill to the right and keep going until you reach a wide room where you can roll to the side just before a boulder crushes you.
The way this room works is that there's a pedestal at the center. A boulder drops onto the pedestal, some gears turn and then a log extends from the far wall and knocks the ball like a pinball so that it rolls down through the trough. There are openings on each side of the chamber. You can head around the platform to the opening on the opposite end of the room (the one below the wooden pole that sends the ball rolling) to find a wooden pole protruding from the platform. Interacting with this will push it in the direction that you are facing, turning the platform a quarter of a turn and impacting the direction in which the boulder will roll. Push it around three times and let a boulder fall and roll each time (to produce a new opening near the beginning of the stage), so that if you look at the pedestal from the direction where you first entered the room, the wooden pole that you pushed extends from the pedestal's right side. This positioning causes the boulder to roll through an opening and fall into an abyss overlooking a bridge, preventing it from being a serious threat to you as you explore the dungeon in the short term (though eventually the positioning will reset).
Now that the boulder won't be bothering you, it's time to explore. There are two main directions worth following. One is positioned just to the side from the opening that you used to enter the room. The other is in the direction that the wooden pole extending from the platform should now be facing if you've followed the above directions.
Start with the latter option. As you start downhill, you should notice some steps leading up to the right. Climb those steps to arrive at an archway. There's a door to the right, but you can't open it from this side. Head left over the archway to another similar room, but in this case some stairs descend toward the left. Start down those to reach a small, square room. There's a door here along one edge with a lift that goes up and down. You need to be careful here. If you step onto the platform as it pauses in front of you, it'll carry you up to a spiked ceiling. So watch for it to come down and drop onto it as it descends. Then when it reaches the base of the shaft, quickly roll off to the side.
In the room that you find at the bottom of that shaft, there's a treasure chest. Do not open the chest, which is a trap, unless you want to defeat the mimic and receive the Lightning Spear that it will drop. Instead, head around it and through an opening. You'll come to a 'T' in the path where you can head left to the top of a ladder or into a hole in the wall that has been produced because of the turning you did on the pedestal in the room at the top of the hill. For starters, ignore the ladder and walk into the opening.
Quickly veer left if another boulder is following you (quite possible, regardless of where you left the pedestal's positioning in the upper room) and let any boulder roll past you. Then check the corpse hanging from a window along the right wall. You can check it to obtain the Covetous Gold Serpent Ring.
Along this path, you can also drop off to the side where you see a ledge with some scaffolding and an enemy to find the Large Soul of a Proud Soldier. Then you can explore the adjoining rooms to find another corpse where you can obtain the Black Sorcerer Hat, the Black Sorcerer Cloak, the Black Sorcerer Gauntlets, the Black Sorcerer Boots and Hush sorcery. You can also find a door that you can open from this side to create a shortcut from the other side.
In the other opposite direction from where you dropped down, there's a detour for the especially brave player. You can head through an opening in the wall, drop down to a ledge and explore it to find the Slumbering Dragoncrest Ring on a corpse. Then you can drop down along the bars that run along the side of the chamber to gradually descend to the lowest point of the area. Defeat the monster that waits there and you can receive the Demon Titanite. Check the room it was guarding for a Soul of a Brave Warrior and for the Scythe weapon. You can roam around this low area and kill another of the large monsters, as well. They're dangerous, but you can attack from a distance with fire and probably do fairly well for yourself as long as you avoid any of the other snake knights. Along the side of the area, you'll also find a high ladder that you can climb to a ledge where you can pillage a corpse for the Large Soul of a Proud Knight. Keep climbing from there while battling any more snake knights that may be chasing you and you'll emerge on the rooftop. From where you appear, you can head forward to attack another giant, or hit him from a distance with arrows to gradually kill him. He drops the Titanite Chunk when defeated.
Next, it's time to return to the room with the pedestal. Get there by whatever route works best for you (trying to run uphill probably isn't it). This time when you arrive in the room where the boulders drop onto the pedestal, you'll want to head through the room to the side of the chute for the boulders. Proceed in that direction and you'll find traps along the left wall. Walk along that wall with your shield facing it. You'll trip a trap, but your shield can block any darts that issue forth from the holes. From there, continue forward and then left through another room.
You'll soon come to a narrow ledge that leads across a wide gap. This ledge is extremely dangerous. Pendulum scythes swing back and forth across it and they're capable of easily knocking you off the ledge and to your death. Their placement can be difficult to track and they're placed closely together. The best way through is to perform two consecutive rolls as the first one finishes gliding in front of you. You should just barely beat the series of pendulums as you reach the far side of the ledge.
When you venture into the next room, you'll find that it has reached a 'T' of sorts. There is a recess to the right where one of the snake knights is positioned (he'll likely come out to attack you as you arrive) and to the left there is a lower path and a staircase. There's a magic-wielding knight at the top of the stairs. Once you start left, begin with the lower route and you'll find two of the Large Titanite Shard on a corpse in that direction. The corpse is guarded by another snake knight, of course. Then you can return and start up the stairs. Take out the magic user with magic or projectiles of your own, or by quickly rushing him.
From the top of the staircase, head left and up more stairs. At the top, look out to see another of the narrow ledges with pendulums. Don't start along it just yet, though. Instead, look left. There is a ledge there with windows along it, and a magic user standing in one of the windows. It will try to pelt you with magic if you let it, which would make crossing the platform difficult, so take care of it with projectile weapons or magic. Then you can more safely creep carefully across the ledge.
When the time comes to pass through the next set of four pendulums, your best bet is to take them two at a time. Walk through the first group and then stop short of the second group as they cross your path. Walk through that second group also, and to the far side of the room where stairs ascend to your right to reach a mist-shrouded gate.
Step through the gate and you'll find yourself on the area rooftop. Ahead of you, there's a giant standing next to a pile of explosive balls. He'll pick them up and chuck them at you as long as you're not behind shelter (and even once you take shelter, if he's spotted you). Head forward and climb some stairs that lead to your left. Continue climbing until you're higher on the rooftop, where the explosive boulders tend to land. Look toward the right and you'll notice a low point along the wall. Head there (not at a run) and carefully look down over the edge of the wall. There's a ledge below you that you can jump down to in order to find the area's bonfire point. Unless you don't want to save your progress up to this point, there's no reason not to use it.
From the bonfire, head through the nearby opening. You'll find yourself on a ledge where you can drop down and onto a staircase. It's the one you climbed a short time previously in order to first reach the rooftop. Climb it again and this time when you reach the rooftop, you should head to the right.
As you proceed to the right, you'll eventually start down some steps toward a lower portion of the outside area. Here you need to be on your guard, as one of the caped knights most common in the Undead Parish area will try to kill you on the stairs. Defeat him in the usual manner, descend the stairs and continue forward through the stone area. Beyond a gap in some high columns, the path turns right and you'll have to battle another caped knight before you can claim the Flame Stoneplate Ring from a treasure chest.
Now return to the rooftop, near where you dropped down to light the bonfire. Proceed beyond that to find the Large Soul of a Brave Warrior on a corpse ahead of some stairs. Climb the stairs to find two Large Titanite Shard on another corpse.
At this point, you can either climb higher onto the roof, or you can follow it toward the right on the current level. It's worth heading to the roof first. There, you should look ahead and notice a gap between your current location and a grated area. Then look left. You'll be able to continue along the high stone ledge and then to the left, there's a wide gap you can leap across. If you fall, you'll land in a courtyard area below where you can grab the Sniper Crossbow and Sniper Bolt from a corpse, so even then your exploration isn't a total loss.
If you make the jump in one piece, however, you'll find a man in soldier gear. He's not an enemy, so talk to him and he'll offer to sell you gear. He carries pieces of the Catarina armor, which are expensive but great defensively. He also has some powerful rings, if you have the souls necessary. Note that besides talking to the merchant, you can also descend some stairs and at the base of the tower (it's a long descent), you can find a magic-wielding snake knight. Defeat him, then check the corpse behind him to obtain the Cage Key, which you can put to use elsewhere in this dungeon. Then you should backtrack to the top of the tower, where you were avoiding explosive boulders a moment ago.
Still on the highest point of the rooftops, jump back over to the main portion of the high stone ledge and run along it toward the far side. You'll find a split in the path where you can head either left or right. Go right first to find a room with some hanging cages. Open one of them with the key that you just obtained and step inside. You'll create a shortcut that leads back to one of the earliest points of the dungeon, the first major ledge with the swinging pendulums. That's convenient, but for now you should go ahead and return to the room where you activated the shortcut. You can continue past it to reach the back side of an outdoor area.
As you ascend through this area, you have to dodge daggers that are being tossed toward you by enemies on the higher wall. The easiest way to get through is to just walk along the ledge with your shield up. Reach the stairs, climb them and then take out any enemies. Past them, you can enter a chamber where you will see a mist-shrouded doorway. That door leads to the boss area, but you shouldn't go there just yet.
Near the mist-shrouded door that leads to the boss, you'll see the base of a spiral staircase. You can ascend it to reach the area where the giant who throws rocks is positioned. You definitely want him out of business. When you get near his feet, he's likely to notice you and will try to stomp you. Dance back and after he sends out a shockwave with his tantrum, he'll collapse for a moment. That frees you up to hack away at his head for a bit. Then you should let the process repeat. You can retreat a few steps down the stairs to stay safe (for the most part), so the giant is actually easy to defeat and when you win you'll obtain a Titanite Chunk as a reward.
Now you're ready to descend and enter the doorway to face the boss.
The Iron Golem is a heavily-armored foe, every bit as durable as his name suggests. However, you can chip away at him just like any foe as long as you can keep out of the way of his fearsome attacks.
If you try to fight at a distance, you'll find that the Iron Golem has ridiculous range. He attacks with an ax that might as well be a projectile weapon. You can dodge out of the way, and you'd better hope you do or you'll take a lot of damage. Mostly, you'll have better luck if you move in close to the golem, but even then there's the problem of dealing with his fists. If you are in front of him at the wrong moment, he'll grab you in his massive hand and crush most of the life out of you before tossing you to a short distance.
To win the fight, you should make sure that you always know where your foe is (through targeting) so that you can see his attacks coming and avoiding them. Always knowing where to expect danger from means the battle is half won. If you want, you can circle and pelt him with fire attacks to drain some of his life, then move in close and finish things off by dancing around his feet. Roll through them and try to keep up an attack from behind so that you're less likely to be grabbed. Make sure that you don't roll off the roof and fall to your death, though. It's surprisingly easy to do so in this battle.
One final note: if you have powerful fire magic and enough of it, you can win this fight entirely by circling your foe and pelting him with fire magic. He's ridiculously weak against that element. Keep it in mind if you're having trouble winning through other means.
Once you defeat the boss, you'll receive the Core of an Iron Golem and a ring of light appears on the ground. You can investigate it to travel to a new area, Anor Londo, by way of gargoyle. It's possible to travel between the two areas at will now. Before you head in that direction, though, you may want to instead start back the way you came. There's still some exploring to do in the local area.
Retrace your steps to reach the place where you found the two titanite shards on the corpse, up a few sets of stairs from where you can drop down to reach the bonfire. If you keep to the lower route instead of heading up more stairs when you came this way previously, you'll be able to follow a lengthy walkway until finally you're able to head down some steps and along a pathway where a heavily-armored enemy (one with a body-high metal shield like the one you fought in the Undead Chapel) waits. If you have powerful fire magic, you can back away and pelt him with it from a distance to eliminate the enemy. Otherwise, you'll want to try to get around behind his shield and hit his exposed flanks.
Past that enemy, you'll find the top of a ladder. You can descend it to find a pathway that leads to the side of a tower. As you start up the tower, you'll be attacked by an armored enemy. He can be tough, but he'll go down quickly if you keep your distance and pelt him with fire attacks. He will drop Ricard's Rapier when defeated and then you can climb the tower that he was guarding. At the top, you'll find two treasure chests. You can open them to find the Divine Blessing and the Rare Ring of Sacrifice.
Now you're almost done in the area. Head back down to the lowest area, where the boulders fall onto the pedestal (you likely remember it well). Make sure that you let boulders descend in all directions so that they break through a wall or two. Then descend along one of the paths and you'll find yourself in one of the first rooms in the dungeon. There's a hole in the wall, though. Pass through it and you'll find hanging cages like the ones that you activated elsewhere to trigger a shortcut.
In one of the cages, you'll find a man named Logan. Use the Cage Key to free him and he'll promise to reward you when you meet him back in Firelink Shrine. In another cage, you'll find a corpse that you can investigate to obtain the Soul of a Hero. Now you're done exploring and should resume your adventure.