But evil things, in robes of sorrow,Assailed the monarch's high estate. (33-34)
This is the first sign of trouble in the poem, the first hint that the happy world of the palace isn't going to last. Something is attacking the palace-head, but it isn't quite clear what yet. We're sort of creeped out by how fast this happens. We were just getting into the rhythm of the happy sections, and suddenly that all gets swept away.
Shall dawn upon him desolate!) (36)
This is a great word to describe the madness of the king. Someone who is "desolate" is miserable, alone. What better way to describe a mind taken over by madness? That feeling is reinforced by the fact that he's somehow outside of time now—as the speaker puts it, the poor king will never see another "morrow."
Through the red-litten windows see (42)
The windows-eyes that were once "luminous" (18) are now terrifyingly red. It's not just that they've turned red (this isn't just a case of allergies!), it's that they are lit with red. They're glowing red from the inside. That's bad news, another sign of the terrible, tragic disorder that has taken over the palace.