It's only occurred to me by a random thought, but there are three Walls, just like how there are two walls and three rings that make up Ba Sing Se, and the kingdom of the respective locales reside in the innermost Wall and ring.
Walls Maria and Rose would be comparable to the Ba Sing Se's Outer Wall while Wall Sina would be comparable to the Inner Wall.
Wall Maria can be like the Lower Ring, Wall Rose the Middle Ring, and Wall Sina the Upper Ring.
And just like AoT's Walls, the Ba Sing Se walls insure protection against outsiders and installing order for the people inside.
Do you see what I mean? --Aggression25 (talk) 07:50, May 21, 2015 (UTC)