This achievement can be unlocked after defeating the Iron Golem in Sen's Fortress.
Upon defeating the boss, a ring of light will appear in the center of the area. Interact with it to trigger a cutscene, after which you will appear in Anor Londo and the achievement will unlock.
Before fighting the Iron Golem, be sure to go up the spiral staircase near the boss fog door to defeat a giant so that he doesn't pummel you with bombs during the boss fight.
Also note that Iron Tarkus' summon sign appears in a small room to the right of the boss fog door.
As always, refer to
http://darksouls.wikidot.com for specific information relating to items or locations.