Exit the platform that houses the bonfire, turn left and kill the Retainer down the hall. Inside these walls, Retainers aren't content to sit contemplatively. They're your enemies.
Head to the opposite end of the hallway and pull the lever to open the door. There's a small hallway that separates you from the next room, which is filled with two Retainers, two Sorcerers and a Crystal Sorcerer.
Lock onto the Retainer standing to the right of the doorway and kill it using melee or projectile weapons. Doing so should convince a Sorcerer to charge, so have him to follow you into the hallway, where you can fight him in relative safety.
Reenter the room and kill the second Retainer and the Crystal Sorcerer. Ignore the items encased in ice for now.
Like many areas in Crown of the Ivory King, there are items and paths that are blocked by ice. Don't worry about them for now. We'll be back.
Walk through the room, turn right and and ascend the staircase. At the top, you'll be notified that you've been invaded by Holy Knight Aurheim. He's nothing to worry about just yet.
Pillage the corpse next to the second staircase for Dried Root x 2.
Head up the second staircase to a room where you'll have to fight two Crystal Sorcerers and a Retainer. Get the attention of a Crystal Sorcerer and coax him down the staircase where you can fight him one-on-one. Repeat the process for the second Crystal Sorcerers and then kill the Retainer. If both Crystal Sorcerers follow you, just keep backing up and heading down stairs until you can create separation and fight them one at a time. When they're down, head back up to dispatch the Retainer.
In the room where the recently dispatched trio first appeared, exit through the doorway that leads to a staircase, but don't head all the way down the hall. Instead, turn right and ascend a second staircase. Eventually, you'll see a wooden barrel following you. That's Holy Knight Aurheim, who's come to battle with a sword and a shield. He doesn't put up much of a fight, and you can defeat him on the staircase rather easily.
Now head to the end of the hallway with the staircase and open the treasure chest for Lloyd's Talisman x 10, an item that will let you play the same trick as the late Holy Knight Aurheim did and blend into the environment.
Head back up the staircase and hop of the ledge at the top to return to the road that takes you to the Outer Wall bonfire.