If a DM’s got to describe the place with words, the closest prime-material analogy is an auto tire. Imagine a tire - no hubcap or wheel rim - lying on its side. Sigil would be built on the inside of the tire. All the streets and buildings would fill the curved interior. Meanwhile, on the outside there’s nothing, see?
One thing this means for describing the place is that, no matter where a cutter stands, if he looks up he’s going to see buildings overhead. Most of the time a basher’s looking across the center of the ring, so he’ll see a broad panorama of the city in the distance (unless, of course, it’s obscured by smoke, smog, fog, or rain). Locals get used to having the gray arc constantly hovering overhead; in fact, the open sky of a normal world sometimes unnerves them.
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.
