The abbey was amply provisioned. With such precautions the courtiers might bid defiance to contagion. The external world could take care of itself. In the meantime it was folly to grieve, or to think. (2)
The use of "folly" in this spot is ironic. Isn't it ridiculous to believe it's folly to think? Yet that's exactly what Prospero's followers advocate, because if they spent time thinking, instead of enjoying themselves, they would be troubled.
But these other apartments were densely crowded, and in them beat feverishly the heart of life. And the revel went whirlingly on, until at length there commenced the sounding of midnight upon the clock. (8)
The image we get of Prospero's party is one of wild and uncontrolled revelry. The narrator connects that revelry this to life, in opposition to death, which it ignores. It's the gloomy chiming of the clock – a reminder of the passing of time – that stops the revel.
It was toward the close of the fifth or sixth month of his seclusion, and while the pestilence raged most furiously abroad, that the Prince Prospero entertained his thousand friends at a masked ball of the most unusual magnificence. (3)
Prince Prospero's most lavish celebration of life occurs just when the Red Death is raging "most furiously" abroad. It' s almost as if he's deliberately – and foolishly – tempting fate.
In truth the masquerade license of the night was nearly unlimited; but the figure in question had out-Heroded Herod, and gone beyond the bounds of even the Prince's indefinite decorum. (9)
On one level, the appearance of someone in a Red Death costume is the height of folly. Whoever it is appears to be making a joke out of something that even Prospero and his friends are unwilling to laugh at. Dressing up as the Red Death? Not funny. Offensive.
Who dares?" he demanded hoarsely of the courtiers who stood near him --"who dares insult us with this blasphemous mockery? Seize him and unmask him --that we may know whom we have to hang at sunrise, from the battlements!" (11)
Prospero's reaction seems rush, impulsive, and extreme to the point of foolishness. He may be a passionate man and an imaginative artist, but does he ever think?
But the Prince Prospero was happy and dauntless and sagacious. When his dominions were half depopulated, he summoned to his presence a thousand hale and light-hearted friends from among the knights and dames of his court, and with these retired to the deep seclusion of one of his castellated abbeys. (2)
This description jars completely with the preceding paragraph about the Red Death "devastating the countryside." Prospero and his friends don't seem to care one bit. They're feeling "happy," "hale," and "light-hearted." They just want to keep having a good time, and refuse to take death at all seriously: the classical image of folly.
But when the echoes had fully ceased, a light laughter at once pervaded the assembly; the musicians looked at each other and smiled as if at their own nervousness and folly, and made whispering vows, each to the other, that the next chiming of the clock should produce in them no similar emotion; and then, after the lapse of sixty minutes, (which embrace three thousand and six hundred seconds of the Time that flies,) there came yet another chiming of the clock, and then were the same disconcert and tremulousness and meditation as before. (5)
The dancers and musicians dismiss as "folly" their nervousness at the chiming of the clock. There are two levels as to what's going on here. On the surface, it might seem silly to get jumpy on account of a clock – they're right to laugh it off. But on a more symbolic level, the clock represents death. What they're trying to laugh off as foolishness is their own fear of death, which makes them foolish. It's telling that, in spite of themselves, the partiers are not able to control their own nerves.
The Prince had provided all the appliances of pleasure. There were buffoons, there were improvisatori, there were ballet-dancers, there were musicians, there was Beauty, there was wine. (2)
Among the "appliances of pleasure" that Prospero packs his palace with are two classic symbols of foolishness: wine, and buffoons (also called "fools"). It's interesting that they're mixed in with the "high" arts.
And now again the music swells, and the dreams live, and writhe to and fro more merrily than ever, taking hue from the many-tinted windows through which stream the rays from the tripods. (7)
After the clock makes everyone nervously stop in their tracks, they always go right back to what they're doing. Right back to the drunken pleasures of life, in other words. The image seems symbolic of their refusal to take death seriously.
It was then, however, that the Prince Prospero, maddening with rage and the shame of his own momentary cowardice, rushed hurriedly through the six chambers, while none followed him on account of a deadly terror that had seized upon all. He bore aloft a drawn dagger, and had approached, in rapid impetuosity, to within three or four feet of the retreating figure… (13)
Prospero's temper leads him to do just about the most foolish (and ironic) thing possible: he chases down and tries to kill Death. That captures the foolishness of trying to avoid death at its absurd extreme. It's no surprise what happens next…