The Horn is merely symbolic, showing us (and Roland) that things can be different this time.
If time didnt turn back, how can there be another time? See this is what I wonder. If the universe hasnt reset except for Roland being back at the begining again....then what? The Tower is already saved. Walter/Flagg etc is dead. The Eddie and Jake that Roland would draw again are dead.
It just doesnt really make sense to me.
Also...and I want to express that I did enjoy the book extremely but...also there were a number of other points I thought were a bit weak.
Mordred was pretty anticlimatic, and linked to that, so was the Crimson King. I'd gotten the impression that the Crimson King was some Sauron-like super baddie and he turns out to be a screechy old guy with Sneetches. Also, in Black House we learn that he's supposed to be trapped in (in fact probably at the top of) the tower, and has been for some time it seems. In the Dark Tower it says he only reccently went there, and seems to indicate he was just a powerful evil king.
We never really find out what the Taheen are. And supposedly the can-toi/low men are human/taheen hybrids, but in what way they are truly different from rat-headed taheen isnt really explored.
I really think Jake and Eddie's deaths seemed a bit tacked on, but it would have been ok if he'd stuck with the 1st ending.
The whole deal with the Prim and the Beams and all is dealt with in a somewhat confusing way. I wish he had stuck more strongly to the theme of the foolishness of trying to replace spirituality with "rationality."
Patrick seemed pretty much tacked on...like he was there only because of the stuff in Insomnia...he wasnt really worked into the plot, just stuck in there.
Walter/Flagg in this book, and his death, seemed like anticlimax. He and the Crismon King were both spoken of previously as being ancient and very peculiar beings and I just dont think that was really followed through.
However, one area that was strong I thought was all the stuff about the todash darkness...he really showed his Lovecraftian roots with that, but in an individual way. The sequence under Fedic with all the doors and corridors with noises and smells and distorations and all, followed by the creature, was very nice.
It was a really fun book but in terms of the end of such a deep and epic series I felt a bit let down.