The Western Sea lies at the edge of the Mohaine Desert. Roland encounters the doors and the lobstrosities here which leads to Roland loses fingers on one hand, making him unable to dual-wield his guns, thus freeing one for Eddie or Susannah to use later on.
The bulk of the second book The Drawing of the Three, takes place on the beach bordering the Western Sea, where a series of doors are located; each one marked to match a card drawn in the tarot reading the Man in Black gave Roland at the end of the first book.
The first door depicts The Prisoner. The second is the Lady of Shadows, Through which Odetta Holmes (and her secondary personality, Detta Walker) are drawn. Roland thinks this may be the three, counting each personality. But Eddie and Roland's actions through the third door depicting Death leads to Jack Mort, who is a psychopath responsible for dropping the brick on the child Odetta, causing the personality split, and later pushing her in front of the subway train, losing her her legs. It is revealed he also pushed Jake Chambers in front of a car, killing him, which is how Jake appeared in the first book in Roland's world.
Mort is killed before he can kill Jake, causing the mental instability of Roland in his world and Jake is his original world as he is now part of a temporal paradox, and Jake's eventual return to Roland's world, completing the ka-tet. The last door is also used to make Odetta and Detta see each other - and with the help of Eddie's love for her, consolidate Odetta Holmes and Detta Walker into Susannah Dean.