The Urth of the New Sun (1987) offers the unique opportunity of showing exactly what we are up against in reading (or deciphering) a Gene Wolfe text. This coda to The Book of the New Sun (1980-83) answers many mysteries of the original tetralogy, but readers should bear in mind that it was not part of the original plan. In a 1990 essay beguilingly titled "Secrets of the Greeks," Wolfe explains the origins of the fifth book:
I had an argument with David Hartwell over this last bit [the ending of The Citadel of the Autarch]. David felt that I should add one more paragraph saying, Okay, Severian went to the universe next door and borrowed the white hole and fixed the sun and everybody lived happily ever after. I, on the other hand . . . felt that a paragraph wasn't going to be enough. David and I yelled at each other for a while, but eventually came to an agreement. David would publish The Citadel of the Autarch exactly as I had written it, provided that I would write another book in which Severian recounted his trip to the universe next door [i.e., The Urth of the New Sun] (Castle of Days, 416-17).
This paper seeks to outline the establishment of four mysteries (two major ones and two minor ones) in TBOTNS and their subsequent solution in Urth. I do not claim these are the only mysteries solved, but I believe they provide different models for study.
Urth gives clear solutions to a number of major mysteries, including the link between Severian and two other men of widely separated posthistorical periods: the mausoleum builder from the Age of the Autarch (whose funereal bronze looks like Severian) and Apu Punchau from the Age of Myth (who looks like the funereal bronze of the mausoleum builder). In the course of TBOTNS it initially seems that Severian might be the third reincarnation of a man who had started as Apu Punchau and then reincarnated as the mausoleum builder--the cultural belief in reincarnation is established early in the text, when Master Gurloes says, "Doubtless I had acquired merit in a previous life, as I hope I have in this one" (I, chap. 7, 76). By the end of TBOTNS, however, Severian has a new theory of a "First Severian," the original version of himself born in his time, a man who went to Yesod and returned as a time traveler, first building the mausoleum in the Age of the Autarch as a message to his younger self (our narrator Severian), then traveling back to the dawn time to become Apu Punchau, ultimately dying in the time-fight against Hildegrin (II, chap. 31).
In Urth both theories are combined and refined, despite the seeming paradoxes. It is plain that Severian will become not only Apu Punchau and the mausoleum builder, but also the Conciliator himself, as well as the New Sun, and even the Sleeper of Ushas. At the same time, Severian is shedding versions of himself, such that he is not, nor will he ever become, the same Apu Punchau who rises from the dead, fights against Hildegrin, and implodes in TBOTNS. This Apu Punchau has a separate life, a different adventure. (Between his rising from the dead and his final implosion, we know of this separate Apu's career as a vivimancer in the stone town, which apparently leads to an encounter with the pelerines at some point, witnessed by his having one of their capes--an artifact he leaves with Severian just before imploding.) So rather than being reincarnations that imply spiritual continuity across linear birth-death cycles, the iterations of Severian are exposed as being time-traveling slivers of himself. Thus Severian becomes like the First Severian in taking actions that affect his younger self, but he remains forever unlike the First Severian in that he is still being shaped by the First Severian.
Urth also delivers an unambiguous answer to a major mystery that was probably not even recognized as a mystery by most readers--the relation of New Sun and the deluge. In at least seven points in TBOTNS, Severian has intimations of deluge, the most emblematic case being voiced on the ride in a howdah through a stately forest, on the way to meet Vodalus: "I feel now that I'm traveling through the Citadel in a flood, solemnly rowed" (II, chap. 9, 76). Urth shows that the arrival of the white fountain in Urth's solar system causes gravity waves to trigger a literal deluge. At the end of his guilt-wracked survey of the destruction, Severian enters the water and re-enacts the vision quoted above:
"Moonlit waves closed about me, and I saw the Citadel below me. Fish as large as ships swam between the towers . . . . this drowned Citadel vanished like the dream it was, and I found that I was swimming through the gap in the curtain wall and into the real Citadel itself. The tops of its towers thrust above the waves; and Juturna [the undine] sat among them, submerged to the neck, eating fish" (V, chap 48, 341).
With the mysteries laid bare, it becomes apparent that enough clues were provided for such major mysteries to be solved in the original tetrology. That Severian would travel through time to become both the Conciliator and Apu Punchau, rather than being a reincarnation, avatar, or descendant. And that the world would drown with the coming of the New Sun.
Urth gives answers to minor mysteries of TBOTNS, as well. For example, the memory-free nature of Meschia and Meschiane in Talos's play "Eschatology and Genesis" seems like a playful riff on a collision between the first day in the Garden of Eden and everyday reality.
Meschia: (Examining NOD.) Why, it's only a statue. No wonder he wasn't afraid of it.
Meschiane: It might come to life. I heard something once about raising sons from stones.
Meschia: Once! Why you were only born just now. Yesterday, I think.
Meschiane: Yesterday! I don't remember it . . . I'm such a child, Meschia. I don't remember anything until I walked out into the light and saw you talking to a sunbeam (II, chap. 24, 213).
The play opens with this comical approach to the end of the world meeting the beginning of the world, where a new Adam and Eve are walking and talking yet are very fuzzy about memories. It isn't until the end of Urth that Severian discovers that Ushas was in fact colonized by memory-wiped colonists:
I recalled what the young officer had reported [as the Deluge reached the House Absolute]: that Hierodules had landed a man and a woman on the grounds . . . Remembering that, it was simple enough to guess who my priest's forebears had been--the sailors routed by my memories had paid for their defeat with their pasts (V, chap. 51, 369).
The sailors who had run away from fighting against him at the trial in Yesod are referenced in two brief passages about thirty chapters earlier:
It seemed that I had no sooner joined the battle than it was over. A few sailors fled from the Chamber; twenty or thirty bodies lay upon the floor or over the benches (V, chap. 21, 155).
As to their fate, Gunnie asks, "Where are my shipmates? The ones who ran and saved their lives?" and Apheta answers, "They will be returned to the ship" (V, chap. 22, 161).
In this convoluted manner, what seemed a simple joke about Adam and Eve in the play turns out to be entirely true and real.
Then there is the mystery of Hethor's pets. The notule, a monster that is like a dark scrap of flying paper, is small and could easily be carried in a pocket. But the blob-like slug and the man-sized salamander are so big that Severian muses upon a cargo vehicle being necessary:
But what of [the slug,] the creature we had seen in the hall of testing? . . . A large cart, surely, would have been required to transport and conceal it. Had Hethor driven such a cart through these mountains? I could not believe it (III, chap. 22, 180).
Later, Severian learns from Agia that the monsters come from magic mirrors (IV, chap. 30, 240). Magic mirrors, in turn, are introduced to the reader in previous volumes.
First Severian retells a story Thecla told him about Domnina's visit to Father Inire's Presence Chamber, where a Fish was forming at the intersection of eight magic mirrors. The girl asks him if this is how the offworlders come to Urth, i.e., by teleportation.
"Has your mother ever taken you riding in her flier?"
"And you have seen the toy fliers older children make on the pleasance at night, with paper hulls and parchment lanterns. What you see here is to the means used to travel between suns as those toy fliers are to real ones. Yet we can call up the Fish with these, and perhaps other things too" (I, chap. 20, 184-85).
Inire's veiled point is that mirrors are used as propulsive sails, not as a teleportation system between star systems. But one volume later, Jonas uses the mirrors in the Presence Chamber to teleport himself away in the style implied by Domnina's question, albeit to another universe rather than another star system (II, chap. 18, 167-68).
Magic mirrors are also related, in a somewhat different way, to the Book of Mirrors, which seems to be a teleportation system from planet Urth to the space near Tzadkiel's starship (II, chap. 21, 186).
As for the mirrors used on a starship, we have Hethor's line about "demon-haunted" sails:
Sometimes driven aground by the photon storms, by the swirling of the galaxies, clockwise and counterclockwise, ticking with light down the dark sea-corridors lined with out silver sails, our demon-haunted mirror sails (IV, chap. 4, 35).
The implausible cart for Hethor's pets, the Fish in the Presence Chamber, that the sails of the starship are mirrors, that the starship sails are haunted by demons, that Hethor is a former sailor, that the pets have sorcerous names like "salamander" and "peryton," etc; from these scattered points in TBOTNS, the reader was supposed to have figured out that the old sailor Hethor summons his pets with a scrap of starship sail cloth, a mirror vastly more powerful than the "toy" in Father Inire's Presence Chamber. In Urth, Severian travels on the starship and sees firsthand the magic of the sails in drawing apports into being. Furthermore, in the fierce fighting on the ship the sails themselves are damaged, so that Severian is momentarily frightened of silver scraps flying around, because they remind him of Hethor's pet notules (V, chap. 15, 106). So we are given evidence of mirror scraps.
(But is it really the case that Hethor uses a bit of sailcloth to summon his pets? Robert Borski strongly believes a completely different theory, arguing in chapter seven of Solar Labyrinth  that Hethor is a shapeshifter who enlists the aid of a few other shapeshifters. So Wolfe's mysteries are still open to further analysis and interpretation—
In this manner, Urth answers many mysteries established by TBOTNS. Some of the mysteries are "known mysteries," for example, the mysterious connection between Severian, Apu Punchau, and the mausoleum builder, while others are "unknown mysteries," as in the case of the memory-wiped colonists of Ushas. The exposure of the unknown mysteries in Urth causes the text of TBOTNS to open up into unexpected dimensions, rather like a solid cube that suddenly expands into a hyperdimensional tesseract.
I close this inquiry with a special note of thanks to David Hartwell, "first reader" of TBOTNS and thus the unelected advocate of all Wolfe readers. Hartwell is a very intelligent man, with a Ph.D. in Comparative Medieval Literature and a taste for difficult texts. He is thus highly qualified, so if he doesn't get it, how could we? He served us all very well by arguing with Wolfe about the ending of TBOTNS and then buying its coda, Urth. We owe him our sincere gratitude.
Thank you, David Hartwell!