Board Thread:Dark Souls/@comment-91.120.90.210-20140616222205/@comment-24912592-20140816051718

Capra only leads to the Depths, which are only there to reach Blighttown. The Master Key allows for entering Blighttown from behind, without visiting Gaping. So Capra is optional.

Same goes for Taurus, you can enter the sunlight bridge via the Darkroot area, if you have the Master Key. Many people (me included) like to avoid this fight altogether later on, because it allows for great PvP in the Burg's first bonfire area.

Sif can be an optional boss, if someone drops you the Covenant of Artorias, I believe?