Has TSR or WotC ever detailed what these meeting spaces might look like? I picture ancient Greek amphitheaters, fallen into semi-ruin, with every possible ambush point and place of tactical advantage removed over the centuries.
I believe they have; I was just reading a lore article about the Outlands on Dump-Stat Adventures a few days ago, and that site coalesces information from published articles and books to make a comprehensible overview of each Planes. See the following four pages from their planes series:
The Outlands, the center of the Great Wheel cosmology where all the Outer Planes rotate around the Spire.
dumpstatadventures.com
Sigil, also known as the City of Doors or the Cage, is the planar metropolis and nexus of the multiverse.
dumpstatadventures.com
Visit the sixteen Gate-Towns that circle the Outlands. Each Gate-Town offers access to one of the sixteen major Outer Planes.
dumpstatadventures.com
Factions are the groups that govern Sigil and are searching for the Truth in the multiverse through their various philosophies.
dumpstatadventures.com
In past editions, powerful Wizards would build/buy their Walking Castles only as close to Sigil as their most powerful spell slot allowed. There are RINGS of magical depletion where on each ring you get closer, a spell level's spell slots are locked, until by the time you're at the Spire, all spell slots are locked and Gods are powerless (save overgods). 2e's transition to 3e had a whole adventure path about restoring the Outlands and Sigil and the Lady of Pain into a more balanced place after the chaos of Planescape: Torment and various past Planescape adventures in late 2e, much the way 4e's transition to 5e had The Sundering that split Returned Returned Abeir to Abeir from Toril and ended the effects of the Spellplague (and seemingly fixed 3e and 4e's shrunken scales so that the Realms would be much further between than those editions suggested they were).
This is the language that their Outlands article says about it, though it doesn't detail what that looks like:
"The Outlands is actually made up of ten layers that circle around the spire every 100 miles, though they tend to ebb and flow slightly by a few miles. At 1,000 miles out from the spire, all magic works like normal, but for every 100 miles you get closer to the spire, powerful spells begin losing their potency and stop working. In the 9th ring, at least 901 miles from the spire and beyond, magic functions completely normally. Once you move 1 mile closer to the spire, you enter the second layer, known as the 8th ring, though there is no physical barrier or sign of such an event, only that 9th-level spells can no longer be cast. As travelers continue to move closer to the spire, additional magic can no longer be cast, this even has a strange effect on the gods, limiting their own power if they journey too close to the spire.
9th Ring - 901+ Miles Out / Magic functions as normal
8th Ring - 801-900 Miles Out / 9th-level spells annulled
7th Ring - 701-800 Miles Out / 8th-level spells annulled
6th Ring - 601-700 Miles Out / 7th-level spells annulled
5th Ring - 501-600 Miles Out / 6th-level spells annulled
4th Ring - 401-500 Miles Out / 5th-level spells annulled, poisons no longer work
3rd Ring - 301-400 Miles Out / 4th-level spells annulled, demigods lose their magical offensive power, conjurations spells no longer function
2nd Ring - 201-300 Miles Out / 3rd-level spells annulled, lesser deities lose their magical offensive power, no creature can reach the astral plane from here
1st Ring - 101-200 Miles Out / 2nd-level spells annulled, all deities lose their magical offensive powers
Spire - 0-100 Miles Out / All magic annulled, deities lose all magical powers
The closer to the spire that someone gets, the more power they lose until they leave the circles, even the gods are restrained by such power. Due to this strange phenomenon, many deities and proxies of opposing powers may meet in the 3rd or 2nd ring of the Outlands in order to come to agreements or talk about plans. No one likes to journey into the 1st ring, as it leaves gods completely powerless, not even their godly immunities to a simple club would function in this ring. Deities only meet here for the direst of circumstances, and not for very long at that."
www.dndbeyond.com
