"Numidium's siege of Alinor: It's not the Brass God that wrecks everything so much as it is all the plane(t)s and timelines that orbit it, singing world-refusals.
The Surrender of Alinor happened in one hour, but Numidium's siege lasted from the Mythic Era until long into the Fifth. Some Mirror Logicians of the Altmer fight it still in chrysalis shells that phase in and out of Tamrielic Prime, and their brethren know nothing of their purpose unless they stare too long and break their own possipoints."

So when MK says that the siege lasted into the fifth, he really means that they still fight it in different timelines right?

And what exactly are "Mirror Logicians"?

They are firewalls.
They refute the Numidum from the first to the fifth.
No wonder they are lost, they are up against a somethng that is the same at all time.

The quote as I interpret it:

The Brass Tower is a solid anchor in time and space, in that it doesn't change. It's like a boulder thrown into a river. Water freely flows around it, so it causes no damage, but people who tend to the river bank may try and dislodge it, hacking away at the river bed and the banks inadvertently causing the real damage. The people are the Aedra. World-refusals are mythic attestations of non-existence, telling Numidium, "No. You're not actually there."

Let's imagine time and space as a river again. Throw something into it, and it will make a ripple. The ripple isn't just in one direction but radial. Time is only linear from the perspective of a linear mind, a mortal mind. Brass Tower's effect was so great that it rippled outward beyond the scope of linear Tamriel. It's effects were felt not only forwards and backwards in time (Mythic to the Fifth) but also sideways and diagonally into other realities. A mirror logician is a mage capable of projecting himself into a different reality. A chrysalis shell would be a pocket reality or a bubble from one reality that can exist in another. Therefor we can imagine Altmer standing in shells that float along amidst other mirror universes. My original logic which I still hold to is that the experience would be taxing on the mind, placing the mage either by design or consequence in a dreaming state or something similar. In effect, the whole ordeal would seem like a formless nightmare. Surely you've had those dreams that are more amorphous conglomerations of emotions and blurred colors than actual sights and events. While the mirror logicians struggle in transreality somniwar with Numidium or possibly the energy and/or spirit of Numidium, they operate like a machine would, going through the actions without knowing why. Here's where the final piece of the quote fits in. If a mirror logician thinks too hard about what exactly he is doing, the dreamlike state will pop. His possible point in time and space within this mirror universe will end, and he will either return to his disembarkation point or cease to exist entirely, a paradox existing within an alien reality; being instantly torn apart by logic.

It's possible the question of Numidium, come to the council floor, broke Aldmeris and raised the disagreeable, merish creeds.

In complement-opposition to other posters' descriptions: Numidium is a god, once activated. Thus, it exists (at a higher gradient?) outside of time, and can intrude anwhere simultaneously with big stompy bootprints. (Draco-) Chrysalis Shells are vessels for transcending [Mundus, time, subgradients?] Thus, Mirror logicians exist on a plane from which they can refute the intrusion of said brass stomps.