Dark Albion is unlocked once you have completed the credits level of the main game. Dark Bowerstone is the fifth level of this second playthrough.
Play thought the level as normal, but be a bit more careful than usual. Best done with a fully upgraded character and the +1 heart and/or reflect damage perk from the inside track - linked to the play a 4 player game achievement.
I'd advise using a ranged or magical character such as Reaver or Garth and avoid combat as much as possible.
As Kennansoft notes, it's best to chose the sewer path at the end of the level, and play the chicken-kicking minigame, rather than take on the level boss.
Can be done in single player.