This is story-related and can't be missed.
After ringing both bells you'll view a cutscene where the big doors next to the Undead Parish Blacksmith will open, allowing you entrance to the trap-filled Sen's Fortress. This is the way to go to advance the story and eventually acquire the Lordvessel.
Push your way through this dungeon and at the very top you'll face the Iron Golem boss fight. After defeating him you'll spot an orange ring in the area he was at, touch that ring and you'll be taken to past the big walls on the very top of the mountain, to the enormous city of Anor Londo, unlocking the trophy by doing so.