It’s been a while since I finished the series, but I’ll have a go at it. Some of this is out of order, and a lot of detail is left out, but anyway…
Susannah, who as you will rememeber was pregnant at the end of Wolves of the Calla (and possessed by a woman named Mia), goes to have her baby. The baby is, for all intents and purposes, the Antichrist, or some such; he’s capital E eeeevil. They call him Mordred, which, OK, whatever.
Shit happens. Father Callahan and Jake ultimately rescue Susannah (Father Callahan is killed, but redeemed, in the process). The gang goes to a place where Breakers - psychics, I guess - including Ted Brautigan from Hearts in Atlantis and Dinky Earnshaw from Everything’s Eventual - have been put to work by the forces of Badness breaking the beams that support the Dark Tower. If they are successful, it is implied, very bad shit will be forthcoming. Roland and company free the Breakers in a bloody gunfight, which they win; at the last moment, however, one of the bad guys looks up from his puddle of blood and fires a dying shot at Eddie, which kills him (Eddie).
OK, so then Susannah does… something, I can’t remember what… whilst Roland and Jake travel to our world. There they meet Stephen King - yeah, seriously, Stephen King - who has apparently created all of them. He’s due to be killed by a runaway truck, which will have disastrous consequences for the world he created (ie, Roland’s world). Jake takes his place and dies; Oy dies somewhat thereafter.
Roland and Susannah continue their trek. More shit happens, some of it interesting and some of it blatant filler. They eventually draw a little boy from the real world, a boy (he was in King’s Insomnia, but only briefly, has the power to affect reality by drawing pictures. Before they reach the Tower, Susannah leaves - she has a chance to bail back to her own world (the boy draws her a door) and takes it.
Roland and the boy arrive at the field of roses outside the Tower. The Crimson King is there, lobbing bombs at them from the balcony of the Tower, but the magic boy erases his existence by drawing and then erasing a picture of him. Roland goes alone into the tower.
We find that Eddie, Susannah, and Jake are all reunited in some alternate version of New York, this time as husband, wife, and son.
Roland climbs the tower, steps through the door at the top, and steps into the Wasteland from the first page of The Gunslinger. He doesn’t remember anything of his quest. And then, the man in black fled west across the desert, and the gunslinger followed.