Just like to chime in and say that the alzabo was one of the freakiest creatures / concepts / drugs that I have ever met in literature. The whole arc with little Severian's family and the alzabo in the mountains was a great bit of horror writing.
Finally finished "Citadel of the Autarch" and concluded this quartet. I'm tempted to go back to the beginning and immediately re-read it and I would if I didn't have so much else to read. Definitely give it a re-read at some point though. I wonder what reading it again is like, knowing what you learn at the end of the series. How many clues will I now pick up upon?