This is a highly literary work of dark fantasy, or properly speaking, dark science fiction, as the world of Urth slowly unveiled to us, with its dying red sun and distant, populated stars from which have been brought back alien creatures and plants, is a possible vision of our own world in the far future.
Severian is a young apprentice in the guild of torturers. As sinister as that sounds, Severian, who has grown up with his guild, accepts it as a matter of course, thinking of his job as being no different from any other. The torturers are just working professionals going about their job of punishment and interrogation in the name of the law. This changes for Severian when he forms an attachment to a beautiful woman brought in for "indefinite detainment." It turns out that her sister is connected to a rebel named Vodalus, a rogue whom Severian had a secret encounter with earlier.
After the prisoner is subjected to a horrible, soul-destroying device, Severian gives her the means to end her own torment rather than dying slowly. This act of mercy is both unprofessional and a complete betrayal of his guild, but for political reasons, they cannot simply kill him. Thus, Severian is sent as "Carnifex," or executioner, to a distant village named Thrax.
The rest of the book is taken up by the beginning of Severian's journey to Thrax - though in fact he never actually makes it out of the vast, future-gothic city where he started. Along the way, he acquires a magic sword and a magic ring (not really "magic," but thematically, same difference), he runs into a brother-sister pair of shopkeepers, falls in love with the sister, finds a crazy girl who falls in love with him, and is challenged to a duel to the death with razor-sharp alien flowers as the weapons.
It's a slightly bizarre setting with clues as to its nature dropped abruptly in the middle of long stretches of descriptive prose. Gene Wolf's lush, languid writing is quite a nice treat compared to the turgid prose of less skilled writers in this genre trying to accomplish the same effect, but it demands patience and you have to pay attention or you'll miss something. This isn't a fast-paced book; the infrequent action scenes seem to move no more quickly than the philosophical dialog or the exposition.
It's enjoyable for those who like their sci-fi with a side of literary, but if you want answers, or resolution, you're going to have delve into the next book in the series, because this is only the start of Severian's journey to the throne (a fact he tells us nearly at the outset), and ends thus:
Here I pause, having carried you, reader, from gate to gate. From the locked and fog-shrouded gate of our necropolis to this gate, with its curling wisps of smoke. This gate, which is perhaps the largest in existence, perhaps the largest ever to exist. It was by entering that first gate that I set my feet upon the road that brought me to this second gate. And surely when I entered this second gate, I began to walk a new road. From that great gate forward, for a long time, it was to lie outside the City Imperishable, and among the forests and grasslands, mountains and jungles of the north. Here, I pause. If you wish to walk no farther with me, reader, I cannot blame you. It is no easy road.