Stanza 6 Summary

Get out the microscope, because we’re going through this poem line-by-line.

And travellers, now, within that valley,
Through the red-litten windows see

Buckle in for this last stanza, because things are only getting worse. Remember the description of the "wanderers" looking though the palace window and seeing dancing and hearing harmonious lute music (17-24)? Well, this stanza is like the nightmare mirror image of that happy little scene.
Now the wanderers are "travellers" (which sounds way less happy and relaxed to us). And now the windows they look up to are filled with red light ("red-litten").
Remembering that this is an allegory for the human body and mind. The windows represent the eyes, so this redness probably isn't a real good sign. Red eyes have pretty sinister connotations, connected to madness, evil, and sickness (and, yeah, not getting enough sleep—which can also feel evil).

Vast forms, that move fantastically
To a discordant melody,

Before, the people looking through the windows saw "spirits moving musically." Now they see "Vast forms, that move fantastically." Maybe that doesn't sound so bad, but when we put it in context, it's more bad news for the palace-head. In this case, "fantastically" doesn't mean what we usually think of today (like great or terrific). It means "unreal," like something out of a fantasy.
That's not a bad thing in itself, but when we combine it with the line below about a "discordant" (inharmonious, out of tune) melody, we know that those vast shapes must mean trouble. Remember that, in the earlier version of this image, the music was referred to as a "well-tuned law." Everything is out of whack in this new version of the palace.

While, like a ghastly rapid river,
Through the pale door

Things are still flowing out of the door-mouth, just like in the good old days, but now they aren't "sparkling" (28) at all. The flow has become a "ghastly rapid river." Before the door was "fair" (26) and now it's "pale."
Poe doesn't have to tweak the words that much in order to completely change the feeling of the scene. Before the door and the things coming out of it were delightful; now everything is just plain awful. Darn it all.

A hideous throng rush out forever
And laugh—but smile no more.

The "troop of Echoes" (29) have turned into a "hideous throng." That fits in well with the general turn for the worse that things have taken. Keeping up with the palace-head analogy, the singing that used to come from the mouth has turned into a hideous mess. The palace-head has been taken over by some disorder, some madness or sickness.
The whole poem ends on a really chilling note. Now the spirits coming out of the head laugh and laugh, but never smile. That's such a powerful image of insanity: endless laughter with no joy in it.
Poe really twists the knife by adding a little pause (in poetry terms that's called a caesura) just before the all-important last four words. If we were still in the past, in the days of the beautiful palace, laughing might be a good thing. You might even feel a little glimmer of hope and joy… right before the poem squashes it in the final phrase. It's bad times for the haunted castle, and it always will be.