The Old Wall of Tai-tastigon is the previous city wall before the newer Curtain walls were built substantially outside the old walls. The Old Wall is in ruins now; when Jame enters the city, the read is described as disappearing "under the remains of a gatehouse set in an ancient wall".
The region between the Old Wall and the new Curtain Walls, the Rim District is an orderly one of broad, straight, open avenues, in start contrast to the labyrinthine chaos of the city inside.
↑God Stalk, Book I: Tatters of Dusk, "Chapter 1: Out of the Haunted Lands" — "For several blocks Jame walked along this open way, and then the road disappeared under the remains of a gatehouse set in an ancient wall. On the other side lay the great labyrinth of Tai-tastigon."