Severian is assigned to keep Thecla company, but to guard against infatuation with a prisoner, he is sent to a brothel in the Algedonic Quarter.
cabochon emeralds (58.8/19) shaped and polished emeralds left unfaceted.
fuligin (59.2/27) that color which is darker than black.
vizier (59.1/26) a high ranking political adviser, in chess, the bishop was originally called the vizier.
Martello (60.3/41) a round tower. This is a reference to the Martello tower in the first chapter of Ulysses.
echopraxia (60.4/44) the name of a specific brothel in the Algedonic Quarter of Nessus, or more likely, a category of brothel where the theme is mimicry. At the one in question, the House Absolute is mimicked as the House Azure.
fiacre (60.8/46) a small four-wheeled carriage for hire; a hackney-coach; a French cab, usually drawn by two animals. The patron saint of cab drivers is Saint Fiacre (died 670). His legend made him a misogynist in life and after death, a possible link to his patronage of those who suffer venereal disease. The cabs got their name because they plied their trade from the hotel Saint-Fiacre in Paris.
blazonings (60.11/49) coats of arms
palfrenier (60.1/49) a groomer of steeds
Algedonic Quarter (60.15/52) "pain and pleasure district," the semi-ruinous section of Nessus in which the Citadel is located. The name probably alludes to the presence of the Matachin Tower of the torturers and the Echopraxia brothel within the area.
omphagist (61.1/54) an eater of raw flesh. Inhabitants of the abandoned districts of Nessus area afraid to call attention to themselves by lighting fires. Omophagia, Greek, means "eating into the belly," is a ritual of holy communion by eating the flesh of sacrificial victims, human or animal, so there is also a hint of cannibalism.
Evidence: an ongoing war (58.7/18)
Evidence: the Citadel is a grounded spaceship of some sort (58.14/24)
Evidence: the sun is near its demise and the world is about to end (61.11/64)
Gene Wolfe, poet and prophet, thought up that idea of a color that is darker than black. No we have Vantablack and other colors even darker! Made with carbon nanotubes, these colors - like fuligin - do not reflect light, permitting almost no shadows. Three-dimensional things look two dimensional. Here is a link to an episode of the most excellent podcast 99% Invisible. This episode details the development of paints that are darker than black.