De acordo com Na gaiola [1] um livro de códigos AD e D 2e, a arquitetura da Sigil é descrita da seguinte forma:
Sigil's a city overwhelmed, barnacled, and encrusted with buildings. With a 5-mile diameter and 20-mile circumference (as officially measured by the Harmonium; in actuality, the Lady can enlarge or shrink the city as she wills, at any time), Sigil's huge, but it ain't infinite. Sure, it's big enough to hold new things for the oldest bloods, but the bizarre soon becomes mundane if a cut- ter sees it often enough. Even the view ain't the usual; almost anyplace a cutter stands, if he looks up, he sees buildings. 'Course, smoke and distance obscure the view across the hollow center, creating a gray arc with a few lights.
Despite the city's size, somehow it still always seems crowded. Tiny spaces that might become servants' rooms or pantries in another city are shops and homes in Sigil, where every square inch must house some of the infinite multitudes. Even the buildings crowd each other overhead, and some streets are cut off from the sky entirely, its dim light pinched out by the towering walls.
No livro de origem da configuração de campanha do Google AdWords e Google D2e "Sigil and Beyond" [2], o designer Zeb Cook descreve a arquitetura impossível de Sigil como esta (usando a voz in-character de um habitante Planar):
Get it right out front: Sigil's an impossible place, especially to primes who go barmy when 2 + 2 don't make 4. A city built on the inside of a tire that hovers over the top of a gods-know-how-tall spike, which rises from a universe shaped like a giant pancake . . . it happens all the time, right? 'Course not, but who cares? Being impossible is part of what makes it fun!
For those logical-minded players, impossibility creates all sorts of questions. There's all sorts of things they could ask, like, "Does Sigil have a night and day?," "Can a berk walk on the outside of the place?," "Where's it get its water from?," and even, "What happens if a cutter jumps off the edge?" Most DMs never, ever, worry about such things because they know it just ain't that important, but some of the Clueless are so touchy about the dark of it all that they'll go barmy just looking around. . .
Cook também aborda a questão do horizonte de Sigil:
Another important thing to remember when describing Sigil is that the city's curved in the opposite direction from most prime-material worlds. On those worlds, there's a horizon because the surface has a convex curve, and a cutter can only see what lies along a straight line of sight. In Sigil, things curve up, not down. Looking down a long avenue, it'll seem like the street's rising in front of a body, kind of like looking up a long hill. Just to make it more confusing, Sigil curves both in front of and behind that sod on the street, so he might feel like he's standing at the bottom of a big hollow nearly all the time. The Cage's a flaming big city, though, and it's crowded tall with people, buildings, and smog. The average line of sight is rarely more than a few hundred feet unless a body's looking straight up, so it's not like a berk's constantly looking at the curve of a bowl all around him. It might be a few hours before the average prime, new in town, realizes that the world ain't flat.
Notas
[1] Wolfgang Bauer e Rick Swan, Na Cage (Lago de Genebra, WI: TSR, 1995): p. 6.
[2] David "Zeb" Cook, "Sigil and Beyond" no Cenário de Campanha Planescape (Lago de Genebra, WI: TSR, 1994): p. 58.