A Theory on the Mechanics of Towers
After-the-fact preface: The idea has recently been floated that the Towers are not nearly as important to the stability of Mundus as the lore community generally believes, and I'm inclined to agree. Rather, I see the Towers primarily as mythopoeia amplifiers, means to define reality. This is made evident in ESO as well as out-of-game texts. If they are related to stability at all, they are related to things like climate and magical defense and ecology and so on, things that keep a culture's way of life viable. The reasoning below applies just as well to this idea.
Often people ask how, exactly, the Towers stabilize Mundus, and why, exactly, the Stones are necessary to maintain that function. I've had answers to these questions fermenting for a while, and I think they're ready to put forth, once again as part of my model.
Mythitecture
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With those two quotes, I think it's clear where I'm going with this. Towers are stories. At first I thought they had to be stories about Mundus, but I turned out to be half-wrong, on further reflection. They aren't necessarily stories about Mundus. They're stories of Mundus. They are structures dictated from within, not from without. You follow? The Towers, collectively, are the emergent AE of Mundus, and all its consituent parts, the Aedra, the Earthbones, the mortals within, are subject to them.
Everything in the Aurbis is a song, a story. That includes Mundus, and the end of Mundus' story is the end of Mundus itself. But stories don't have to end all at once. Plot threads can resolve together, or separately, interweaving or splitting as they will. Therefore, to end the story of Mundus, to kill its AE, you have to resolve each of its major plot threads.
The Stones, then, are anchors, resonant evidence of the relevance of the Tower stories to Mundus' overall arc. Material objects and phenomena eternally at the center of myth, things that scream out, "THIS REALLY HAPPENED."
And note, of course, that the plot thread that constitutes a Tower is not necessarily the same as the purpose for which it was constructed, if it has one. The purpose for which a Chekov Gun is acquired is irrelevant to the role it eventually fulfills in the plot. But, at the same time, it is also possible for the purpose and the plot to overlap.
Resolved and Unresolved
To resolve a Tower is to end its relevance to the ongoing story of Mundus. Its pieces may persist, and may even have further parts to play, but they no longer constitute the resolved plot thread.
Take the Towers in turn:
Ada-Mantia: The first plot thread of Nirn, Convention. Anchored by the Zero Stone, which is held in the Foundation Vault. Unresolved.
Red Tower: The second, Red Mountain, the story of Lorkhan's Heart and its flight over Tamriel, and the mer who followed it. Anchored by the Manifest Heart itself, which beat deep within the mountain. Resolved by the Nerevarine, who set the Heart free from its material form.
White-Gold: The story of the Ayleids, bleeding into the story of the Cyrodiilic humans and the empire they forged. Anchored by the Chim-el-Adabal, the Amulet of Kings, a symbol of the Dragonborn lineage and the right to rule. Resolved by the Champion of Cyrodiil, who aided Martin in shutting out Mehrunes Dagon with the power of the Amulet.
Crystal-Like-Law: The story of the Altmer quest for divinity and escape from Mundus, taking the form of the apparatus by which they sought Dracochrysalis. Anchored by a Person, perhaps Auriel, who succeeded by ascending Ada-Mantia, and whom they sought to follow by erecting Crystal-Like-Law. Resolved by the destruction of Crystal-Like-Law itself in the Oblivion Crisis, shutting the Altmer out from Auriel's path.
Snow Throat: The story of Skyrim and its struggles with dragons, with the form of the Throat of the World, sacred to both the Nords and dragons. Anchored by the Cave, perhaps the Time Wound at its peak. Resolved by the Last Dragonborn, who entered the Time Wound and learned its secrets to defeat Alduin for good.
Green-Sap: Falinesti and its siblings, the story of the Bosmer, who did not build, but grew. Anchored by the Perchance Acorn, which could have been in many trees, and so there were many Green-Saps, and each was all, and they walked and shifted. Resolved, perhaps by the Thalmor, when the Green-Saps stopped walking and shifting.
Orichalc: The story of ancient Yokuda. Anchored by the Sword. Likely resolved by the sinking of Yokuda with the Pankratosword.
Walk-Brass: The story of the Dwemer, who hated the very idea of existence, and sought to overrule it with denial and refusal. Anchored, ironically, by itself and its refusal to let go of its single-minded purpose. Resolved by Jubal-lun-Sul, who convinced it to die, who convinced it to end by convincing it that NO is not the only answer.
Khajiit: The story of Azurah's children, who live by and for the moons. Anchored by the Mane, who conducts them to climb and set the moons right again when needed. Resolved (for now), perhaps by the Thalmor, perhaps by assassinating the Mane.
Talos: The story of Lorkhan reborn, through the myth-echo of the enantiomorph, of his ferocious fight to protect what he loves, Mundus. Anchored perhaps by mortal life on Mundus, his beloved children. Resolved perhaps by the exodus of mortals from the Mundus in the wake of the Landfall event. Or perhaps anchored by the existence of the Third Empire, and resolved by its eventual dissolution.
I will also conjecture that the Hist and Argonians constitute an unresolved plot thread, and thus act as a Tower. There are probably others aside.
Is it any wonder that a Tower falls with almost every game? The plot threads are resolving, sometimes through conspiracy, and sometimes through the natural course of events; that only ever brings about an end. But not the end.