Enter the building as the rock doors ascend.
To your left in the first room is a locked door. You'll need the Eternal Sanctum Key to unlock it. We'll get to this later. To your left in the following room is a Stone Switch to the left of a stone door. Hit it to open the door and pillage a corpse with Soul of a Great Hero x 1 and Dragon Charm x 3.
Turn around and head to the doorway opposite the room. You'll see see a tall ghostly figure looking at a tomb. That's a Sanctum Knight, and he's got a friend to his left. Use a projectile weapon or hop down to destroy the armor that's glowing red in front of him. He'll turn from a ghost into an undead and become far easier to defeat. Do the same thing for the other set of armor and the other knight, then go back out the way you came in. It's easiest to fight them one at a time and also possible, if you're quiet enough, to sneak up silently and backstab them before their friend notices.
Open the chests in the room with the broken armor to collect Destructive Greatarrow x 50, a Catarina Helm, a Soul Vessel, a Titanite Slab and Old Growth Balm x 5. All of the pottery is breakable. None of it contains anything.
Exit the way you came in and continue down the hall, to a room with two soldiers wreathed in poisonous gas and two soldiers playing dead on the ground. Approach the poisonous soldier on your left, and back out into the hallway. He and his poisonous friend will follow, though the sleepers will remain sleeping. You should be able to pick off one at a time. Return to the room, rouse and dispose of the sleepers, mindful of the floor switch that will shoot arrows from the wall (as seen in the image above).
Head down to the far end of the room to open a chest, keeping an eye on the doorways to your left. You may need to dispatch of the Dark Sorceress casting hexes and camped outside of the last window before opening the chest safely. The chest is a boobytrapped, so open it and raise your shield. Return for a Bonfire Aesthetic x 1.
Hop down and kill the poisoned soldier holding a shield. In the second cubbyhole from the left, pillage a corpse for a Human Effigy x 1. There's an archer standing guard through a round contraption on the far wall, and you should step on the Stone Switch on the floor to rotate the object, expose him and take him down with a projectile weapon. Step on and off the floor switch to rotate and create a way through the doorway.
Walk through, turn right, and then turn left at the bottom of the stairs to pillage a corpse with Repair Powder x 1.