Urth Net
(TBOTN) Nessus, Citadel, Wall
Nessus-the-mobile, the Citadel, and the enigmatic Wall.
For simplicity's sake, let's suppose that the citadel is located in the exact center of a circular Wall.
Now then, in Typhon's time (in URTH), the "towers" are still ships. The doctor who checks Sev refers to the "zoetic transport," for example. I'm pretty sure we don't see Nessus in Typhon's era in URTH (we go from Saltus to the Citadel); and in the Long Sun books I believe we have indication that in Typhon's era the Citadel was at the northern edge of the city, rather than being at the southern edge as it is in Severian's childhood.
So my question about where "Nessus" is at any given era relates to where the leading edge and trailing edge are in relation to the citadel complex. The citadel is like an island garrison, and Nessus is a tide that rises up and finally past it.
If there were breaks in the wall, these would serve as access ways; but what we see at the one gate is a tunnel like a mine shaft. Presumably the river passes through a similar tunnel, but Severian is maddeningly silent on this (I think he literally sleeps through it on the Samru).
The Wall is far from the sea.
When I wrote about "gigantism" in gothic, I thought of CASTLE OTRANTO (the giant there being a ghostly arm, iirc), and for genre of course I thought of THE NIGHT LAND. I nearly mentioned hartshorn by name in that regard -- I didn't, but he has popped up anyway. Yay!
More gigantism in TBOTNS: the league-high cliff, the carved mountains, the monster-mammals (baluchitherium, mammoths, et ali), the undines, the thing in the mine at Saltus (sounds big, at least) . . .
Right, the =Citadel= is the one with walls of unsmeltable metal. The Walls of Nessus, who knows? They are said to be honeycombed; basically an anthill of the Autarch's freaky soldiers. Beastmen, anpiels, things like that.
=mantis=