The Shadow of the Torturer: Urth: Book of the New Sun Book 1 (Gateway Essentials 174)
About this deal
The woman says that she can be Thecla for him if Severian will only be "strong enough to master reality, even for a little while. It's implied that the coming of the New Sun will begin a new stage of humanity's evolution wherein something better might be possible.
Although it's barely over 200 pages, it demands a lot of thought from the reader untying the Gordian Knot that is the narrative where you feel like you are immersed in a three dimensional alien puzzle where you don't know the rules and you don't even have all the pieces yet. However, it is likely that this is a subversion rather than simply lampshading an unsatisfying plot development.
Just as all that appears imperishable tends toward its own destruction, those moments that at the time seem the most fleeting recreate themselves - not only in my memory (which in the final accounting loses nothing) but in the throbbing of my heart and the prickling of my hair, making themselves new just as our Commonwealth reconstitutes itself each morning in the shrill tones of its own clarions. Torturers are forbidden to marry (because their wives would be outcasts the same way torturers are, but without the compensations of belonging to a guild and knowing they are a critical part of the government apparatus.
Severian is struck by "how much lighter it was than the true sky, whose blue is nearly black even on the brightest day. Reading this book is like accidentally stumbling into the dreams of another person that is barely keeping their sanity intact. GURPS New Sun, an RPG sourcebook by the author of Lexicon Urthus includes a lot of background hints.However, the slings and crosswbows have pyrotechnic ammo, said lances fire energy beams, and the cavalry ride creatures that can go 100 Miles per hour. This work is also extremely metatextual, allegorical, and metaphorical from a wide variety of cultures, religions, and languages. He wisely gets the hell out of there before he can see what it is, but it's large enough that its footsteps shakes the earth and it scares the man-apes so much that they run for their lives rather than continue trying to kill Severian.