The Dark Tower is the point connecting all worlds, and it's falling apart.
Well, in a manner, and supposedly.
The Tower is a central point for every world. It holds them together, keeping a vauge sort of sanity to it, keeping a form of order. It is the keeper of the Beams, it is the wonder of the universe, the true mystery.
Unfortunately, it is breaking up and decaying. As it does this, the worlds are begining to intermingle, to tear apart, to undo themselves. As it falls, the universe breaks up.
As of yet, no one in Mr. King's books have actually seen the Tower while awake and fully concioucess, though several have seen it in dreams or visions.
They all see the same, and they all knowthe same. Some force is causing the Tower to crumble, and the worlds are going with it. Perhaps it is the Crimson King; in fact, that is the most likely answer. It is the Crimson King that many see against the Tower. It is the Crimson King who loves disorder.
Roland Deschain has set out to keep the order, to restore the Tower. Will he succeed? We know not.