You'll get to Anor Londo after defeating Iron Golem and touching a golden circle on his arena.
From Anor Londo you can return to Sen's Fortress moving to Gargoyle on the wall.
Go down the long stairs.
Turn left in front the large knight - you do not have to fight him now.
Inside the building you'll encounter two more knights - they are guarding the chest with Demon Titanite. You may leave them now, because you'll unlock the bonfire after a moment.
Instead of fighting, go outside with another exit.
You'll get to the junction - first go to the left and down the stairs.
Light the fire once you get down. A person standing next to the wall is local Firekeeper. He won't attack you before you kill boss Gwyndolin (optional).
Go outside. A path to the left, towards a building on a hill, leads to Duke's Archives (this way unlocks only after obtaining Lordvessel).
So go to the only possible way now - to the elevator.
Go down the elevator and then go down the stairs. You'll get to the level guarded by a Gargoyle - this monster is similar to Bell Gargoyle, so apply the same tactics as when you were fighting the boss.
Go back to the stairs and look for a platform located a little bit lower. Jump down on it.
In the backside part of the tower you'll find a chest with Demon Titanite.
Now you have to do something surprising - use the lower platform to go back to the opposite side. You'll see a nearby cathedral and its "bays". One of them reaches your level, so climb on it.
Climb up and watch yourself not to fall down.