Aldia's Keep can be accessed only once you have obtained the King's Ring from the Undead Crypt. To reach the keep itself, teleport to the Ruined Fork bonfire (second in the area). From here head up the nearby stairs and take the door directly next to the top of the stairs. There is a Red Tearstone Ring on a corpse in a pool to the left along the path if you haven't obtained it in a prior visit. At the far end of the path, you will find a door that can only be opened by wearing the king's ring. So do that to pass through. You will enter into a large open area with a path in the middle. Follow the path a little bit until you see a tree to the left with long grass at it's base hiding a corpse holding a Soul of a Nameless Soldier and Petrified Dragon Bone. Before moving ahead, look carefully in the long grass ahead to spot four or five petrify dogs (like every dog enemy in the game, they get near you and then lunge to attack. These can inflict bleed and petrify status as well). Take care of the pack and pick up the items of the two corpses in the long grass: Poison Throwing Knives and Alluring Skulls. Travel through the rest of the grass to the back corner of the wall to find a mimic chest that will drop a Sunset Staff and Dark Mask upon it's demise. Look inside the nearby hut to find the first bonfire of the area. Lucatiel will be next to it if you have talked to her in all the prior locations (if you exhaust her dialogue here, she will no longer appear in the game until you start new game plus). So if you are trying to obtain her equipment by fulfilling the summoning requirements, make sure you complete at least three of the required battles with her summoned and her surviving before talking to her here.
Head back out to the path and look for a set of steps nearby. Before climbing up them, head to their right side to find a body with a Fire Seed on it. Head up the stairs and past the small water fountain. You can drop down into the fountain from above to pick up the Radiant Life Gem from the corpse inside. Hop out of the fountain and keep walking up the stairs. Before going through the door at the top, take a left into the small alcove to find a body holding onto a Twilight Herb. Backtrack to the door and head inside. This will trigger a phantom invasion by Aslatiel (Lucatiel's brother). Despite his lineage, he only performs simple melee attacks and is quite vulnerable to ranged attacks. Melee characters will want to use shields with high physical defense or weapons with a wide swing arc to beat him quickly. Once he is out of the way, head through the next door (ignore the rocking cart as they contain basilisks). Ahead of you is a large metal door that will open to reveal a huge set of dragon bones. When you approach the bones, they will move for a few seconds and then come crashing down towards the door. To avoid this trap, when it starts moving, head back through the door you just came through and wait until the bones vanish.
Now the trap is out of the way, we can take in the room a little. A corpse on the ground here holds a Fading Soul. At the base of the steps, look for the three arches straight ahead. A hallway will branch off to the left and right. A lever down the left path is a double edged sword. Pulling it will free a NPC called Navlaan who will then proceed to invade as a phantom in several locations as well as preventing you from taking a side quest and some loot. Either way, head down the right hallway on the other side and travel through the mist door to meet Navlaan. If you did pull the lever he will sell you some wares. If you didn't and you are hollowed, he will offer you a sidequest.
Head back out to the base of the stairs and ascend to the next landing. Walk to the right side and head towards the room with mirrors on the wall nearby. As you approach, look to the right to see a low wall that hides a chest at the end that contains the Great Magic Barrier miracle. Head back towards the room of mirrors carefully as three phantoms will break out of the mirrors when you get near. You will see them pounding and cracking the glass before they emerge (they are similar to those that came out of the looking glass knight's shield). Once all three are taken care of, loot the corpse leaning against a pillar here for a Northern Ritual Band +2 and Petrified Dragon Bone. Backtrack to the landing and head to the top of the stairs to find a petrified cyclops. Before wasting a branch, you can take a side passage on the left side of the previous landing to find another set of stairs leading up past the cyclops. Up this alternate path, you will see a large basilisk in a cage ahead. You can leave it alone completely and it will only be a threat if an enemy attacks the cage. Alternatively, you can attack the cage, causing it to break and snipe the basilisk from range to have a chance to pick up a free magic stone it can drop. Next to the cage is a body holding Crimson Water. Head to the balcony on the other side to find a chest containing a Bonfire Ascetic. Head back to the cage and walk through the nearby doorway (note the switch in the wall next to this doorway).
Inside the small room is a dragon statue with a chain in it's mouth. Pull the chain to open the opposite door (closing the door behind you). The hallway beyond the small room is very dark and filled with cages containing enemies all the way along (including a cyclops in a large cage at the end). A pharro's device in the wall to the right will activate the lights in this hallway, so decide if it is worth it before using it. The large paintings in this room conceal two dark acolytes who will attack as you walk past or if you attack the paintings hiding them. There is also a corpse her with a Radiant Life Gem on it to collect.
At the far end of the hall, be careful when approaching the large doors as a cyclops will burst through them dealing massive damage if it hits you. After dodging the initial charge, you can lure him over to the chained door on the left side of the hall and have him break it with his attack to reveal a chest containing a Malformed Shell. Be very cautious about letting the cyclops touch any of the cages as he will release the monsters inside (who will promptly attack as well). Once the cyclops is out of the way, head back down the hall towards the room with a statue. As you do, look for a small door on the right wall (looking towards the dragon statue room). Open it up to enter a lab full of glowing green phials and beakers. Descend the staircase nearby, but stop halfway down. An hidden door can be opened here that reveals a bonfire. Back on the stairs, continue down to the floor below. The room here contains a huge pool of acid surrounded by a cage with mutated dogs prowling in it. Kill the dogs with ranged attacks if possible or open the door and lure them out of the cage before killing them in melee. Once they are out of the way, remove all your equipment and head into the corrosive sludge. Pick up the Aldia Key from the first corpse. This key will allow you to open the door on the right in the hallway above. We'll be using it shortly. Loot the Soul Geyser sorcery and Petrified Dragon Bone from the other corpse in the pool.
Hop out of the acid and put your equipment back on before backtracking to the hallway above. Head towards the cyclops end of the hall and enter the door on the right with the aldia key we just picked up. Directly ahead when you enter is an acolyte with his back to you. Stab him in the back or just attack him to kill him off. Another acolyte will be to the left in the same room. A large spore bug will be in a cage at the other end of the room as well. Kill off the bug as well (to make sure it isn't released by the cyclops. Pick up the Soul of a Proud Knight off the corpse the first acolyte was examining. Another body at the other end of the room holds the Large Soul of a Brave Warrior. Open the door at the far end of the room to find yourself standing behind the cyclops (if you didn't kill it earlier). The door to the right opens into a small room with a door that a second cyclops will burst through when you near. Avoid the onrushing enemy and kill it. A corpse against the wall holds Simpleton's Spice and a Twilight Herb. Directly behind the new hole in the wall is the mist door that leads to the area boss.
Boss: Guardian Dragon
Personal Death Count: 0
Bite: The dragon will move it's head towards you in an attempt to bite you. Move backwards or roll away when you see his head coming towards you.
Flame Breath: The boss will rear up on it's back legs and then drop down and release a massive fire stream towards you. He can also perform this attack when hanging from the wall of the cage. He will tilt his head towards you and fire a stream from his mouth. Roll to the side when he drops down to avoid the blast or sprint away from your position if he is using the wall variant.
Fire Bomb: The dragon will fly up into the air and hover for a few seconds before shooting a massive fireball at your location. Roll to the side or sprint to avoid the flames.
Leg Stomp: If you are standing near the legs of the dragon, it will raise the closest one and stomp towards you. Roll backwards to avoid the attack.
Tail Swing: The dragon will swing his tail in an arc if you are standing behind him. When you see the tail wind up, roll backwards out of it's range.
The Guardian Dragon will start off the fight in a heap directly ahead of you. Ranged characters can use the time it takes to rise to take a massive chunk of health off the boss (I managed to take half it's health before it managed to attack the first time). Melee characters will also want to rush forward and get a couple of swings in before the boss is ready to fight. Make sure your fire defense is as high as possible before the boss fight (due to his flame attacks), physical defense also helps too for his basic melee attacks. Melee characters will want to get close and bait his attacks (like usual for most enemies) and then attack during his recovery period.
Ranged characters can either stay in close and use the same tactics as melee or stay back and pelt him with attacks when he uses his fire attacks or flies into the air. If he climbs onto the cage walls, move to the edges of the area and watch his head. If he uses his flame breath, move away to avoid it. If he uses his fire bomb attack, he will target it in the center of the area, so it should easily avoid you. When he lands, make sure you aren't underneath him to avoid taking damage. After a few good hits, the boss should go down.
After the boss is defeated, head through the other door in the enclosure. This leads to a short walkway with a lift at the end. Turn around to spot a corpse holding a Dragon Scale. Ascend in the lift (looking at the view), to reach a circular room. A doorway in the wall leads to a short corridor that leads out to the Dragon Aerie. The guide continues in the that section.