Hila, the portly dwaven lecturer, enters the empty board-room style chamber in the Festhall. She is supposed to be hosting a discussion on planar philosophies, though she isn't quite sure where to begin. Gently stroking her silken beard, she contemplates for a moment and decides that it'd be best if those who enter for the lecture decide on a topic.
She eagerly stares at the door, hoping that someone will abate her thirst for intellectual intercourse.
My guess is as good as anyone's - though I do think you summarize the paradoxes quite nicely: another one is that it sits atop an infinitely tall Spire. I tend to agree that the ring further serves to symbolize the center of the multiverse where everthing comes together, quite literally, 'full circle.'
Another perspective is that we sods are looking too much into it: the city is just shaped as a ring for either or both of two simple functional purposes: 1) to be able to rotate around the spire, and 2) to keep the inhabitants from falling out (as they would more easily if Sigil was shaped as a hollow cylinder, for example). Every other interpretation could just be speculation.
A woman leans forward from the shadows. She is young, but not. Her medium-length hair is grey. From one side of her back sprouts a black, demonic, bat-like wing. The other side has an angelic wing with white feathers. She is dressed in Sigilan fashion, but favoring darker leathers. She has a single earing, in the symbol of the Bleak Cabal.
"Unity of Rings, and Rule of Three, are not the same. Rings are essentially duality, not trinity. Inside-Outside. Our friend has already pointed out many paradoxes of rings and of Sigil, but they are each dual paradoxes. I have one more. Sigil - Undersigil. I find it interesting that I've never talked to anyone who's seen Her Serenity in Undersigil. Of course, enough berks disappear that those who do encounter Her, don't come back.
Sigil, then, is a plane (of sorts), that exibits the Unity of Rings stongly. What does this mean? Why nothing, berk, that's just the way it is. Why is it like this? See previous answer.
If the city is meant to serve function, then this still raises the questions; Who made it this way and for what motive? I suspect we ourselves make it this way. We are restrained by the moral laws of the multiverse, and deceived by the hollow philosophy of the factions. Sigil is not just a cage that keeps the Gods out, it is a prison built from our own minds keeping us safe from the things we most desire but are too weak to embrace. Threats of damnation and promises of eternal bliss make up the walls of our prison. The Lady is our warden, casting shadows of fear across our minds. Eventually, someone will break free of this prison, and when they do, all of Sigil will tremble.
An elf enters with a series of skips and out of sync strides, for an elf he is noticabley worn, but the gleam in his eyes and smile on his lips show a kind of vibrance most Sigilians are lacking. He wears the robes of a monk and carries, as far as you can tell, no equiptment or material wealth.
He is Brother Peel.
"Yeah, that," he pauses, chews a spare flick of cotton from his robe and carries on, "Or mahbe the Dabus and the Lady make it this way, an' we just live here. Proper colonial mind set that, that we own the place when it's obvious there are things here that were knocking about before us!"
"Hows this for a reason, chaos. Oh come on hear me out!" he huffs and stands quickly, swaying on his heels slightly, "Like Miss Bleaker over there, I think the city just is, but unlike her I think its quite a happy accident! If things had gone differently, the multiverse had formed a mote out of place, we'd be down an infinite spire and a donut city!"
"It could be a ring because that's just one of the natural shapes of the multiverse, something writ down on the very base material of it when it first started. It was a random chance, couldda been the planes where all square, or or spheres! Picture a plane all of spheres, sphere world, sphere moon, sphere stars. Ha! Hilarious."
The elf blinks a few times, sits down on a table edge and places a bare foot across his knee and begins to massage some life into it,
"Also, to the Bleaker, I know a bloke named Danny the Grue that once saw her Daggeriness in Undersigil, so, there is that, eh?"
Why a ring?
This is certainly one of Sigil's greatest oddities. We often call it the Cage, and perhaps this is no coincidence. Rings are, after all, rather bounded things. Infinite but closed. Filled with promise but binding in intention. Some say the multiverse has no center, but if you were to ask the average cutter where such a center might be if there was one, most would say Sigil without giving it a second thought. Since we are in the very center of the multiverse we are in some sense amongst the Gods, and yet we have a place that defies them, a prison by exclusion, but yet at the same time this prison provides portal to thse very same who are defied. One has to ask if this isn't one great riddle posed on the grandest of scales. While all these factions go running around developing rather absurd philosophies, I sometimes wonder if the answer doesn't stare us right in the face. So tell me, Hila, why a ring?