There are two ways you can go when your in the lost Bastile depending if you came through the bird nest or new mans warf. If you came in through the nest you follow the path it will lead you to a room you can open which gives you the key. New mans warf way, you can't proceed until you get the key from the other side, this allows you to skip 1 boss if you don't have a fragrant brach of yoire. There should be a shortcut allows you to go to the other side.