Chaos is hard to get to - it is out of normal Trump range (though there are Trumps which allow people to get there and back, and it is possible to go via several 'bridging' Trumps), and it takes several months of shadow shifting through many dangerous and unstable shadows (on the other side of Ygg) to reach the Abyss.
The last barrier is a range of mountains, beyond which lies a plain the other edge of which is the Abyss itself. This plain, particularly near the edge of the Abyss, is dotted with large metal fortresses assembled there by forces from Amber to keep watch on the Courts.
"Where? The senses are such uncertain things, and now mine were strained beyond their limits. The rock on which I stood ... If I attempted to fix my gaze upon it, it took on the aspect of a pavement on a hot afternoon. It seemed to shift and waver, though my footing was undisturbed. And it was undecided as to the portion of the spectrum it might call home. It pulsated and flashed like the skin of an iguana. Looking upward, I beheld a sky such as I had never before set eyes upon. At the moment, it was split down the middle - half of it of deepest night-black, and the stars danced within it. When I say danced, I do not mean twinkled; they cavorted and they shifted magnitudes; they darted and they circled; they flared to nova brilliance, then faded to nothing. It was a frightening spectacle to behold, and my stomach tightened within me as I experienced a profound acrophobia. Yet, shifting my gaze did little to improve the situation. The other half of the sky was like a bottle of coloured sands, continuously shaken; belts of orange, yellow, red, blue, brown, and purple turned and twisted; patches of green, mauve, grey and dead white came and went, sometimes snaking into belthood, replacing or joining the other writhing entities. And these, too, shimmered and wavered, creating impossible sensations of distance and nearness. At times, some or all seemed literally sky-high, and then again, they came to fill the air before me, gauzy, transparent mists, translucent swaths or solid tentacles of colour. It was not until later that I realised that the line which separated the black from the colour was advancing slowly from my right while retreating to my left. It was as if the entire celestial mandala were rotating about a point directly overhead. As to the light source of the brighter half, it simply could not be determined. Standing there, I looked down upon what at first seemed a valley filled with countless explosions of colour; but when the advancing darkness faced this display away the stars danced and burned within its depths as well as above, giving the impression of a bottomless chasm. It was as if I stood at the end of the world, the end of the universe, the end of everything. But far, far out from where I stood, something hovered on a mount of sheerest black - a blackness itself, but edged and tempered with barely perceptible flashes of light. I could not guess its size, for distance, depth, perspective were absent here. A single edifice? A group? A city? Or simply a place? The outline varied each time it fell upon my retina. Now faint and misty sheets drifted slowly between us, twisting, as if long strands of gauze were buoyed by heated air. The mandala ceased its turning when it had exactly reversed itself."
...
"Once again, I beheld the chaotic colours. From my right, the darkness herded them. The land they danced above was rock-cropped and cratered, no sign of any life within it. Passing through its midst, however, from the far horizon" "through a broad pass in the mountains" " somewhere to the right, inky and serpentine, ran what could only be the black road."
'The Hand of Oberon', Chapter VI
Over the Abyss (also known in Chaos as the 'Sea of Stars', from its appearance) hangs the black mass which is the Courts of Chaos, linked to the edge of the Abyss by the seemingly insubstantial shifting bridges known as 'filmies':
"As I continued to stare, fascinated, across the chasm, it was as if my eyes adjusted or the prospect shifted once again, subtly. For now I discerned tiny, ghostly forms moving within that place, like slow-motion meteors along the gauzy strands. I waited, regarding them carefully, courting some small understanding of the actions in which they were engaged. At length, one of the strands drifted very near. Shortly thereafter I had my answer."
'The Hand of Oberon', Chapter VI
Inside, the Courts are a three-dimensional labyrinth of glistening black and red buildings closely packed and intertwining almost organically around tunnels, halls and plazas, with planes and angles which seem just ... wrong. The material of the place shifts and wavers when looked upon, though remaining solid (like the plain at the edge of the Abyss), and this has had serious effects upon susceptible minds. Balconies and windows look out over the Abyss from the belly of the city, a highly disturbing view to one of Amber. The labyrinth that is the Courts seems to shift and change from time to time, when no-one is looking. Soldiers have been lost in these shifts.
The entire Courts of Chaos exude an air of vast age.
Society in Chaos seems to be divided into various noble Houses or families, who dwell in realms knitted together from shadows and parts of shadows, and, apparently, very well defended. At least, well enough to render Fiona catatonic for more than 100 years.
When Amber forces, led by Benedict, took control of the place, the people of Chaos retreated into their private realms, and have not been seen outside of them since then. No troops are stationed inside the Courts any more. The first troops stationed inside became violently psychotically insane within a day, incurably, and even now soldiers go mad from time to time (this does not seem to affect those of Royal blood, although no-one goes into the Courts for too long, just in case). The madness seems to be partly an effect of the shimmering material of which the Courts are made. Even in the forts on the plain at the edge of the Abyss, a week or so of constant duty there is enough to send a soldier mad. For this reason troops are regularly rotated out to bases in more hospitable shadows nearby.
From Merlin, who really only seemed to know about the Royal House in which he was raised, the rulers of the Courts, a families realm can contain whole worlds, often with wildly varying conditions, environmental and magical, within them, knitted together by or around something very similar to a Black Road which loops and twists through multiple dimensions and links back to its starting point (eventually). Fiona and Bleys, who allegedly had contacts in Chaos during the War (though not, of course, any more, oh no), claimed they knew nothing more than this.
Apparently the majority of the inhabitants of Chaos are demons and similar non-human creatures:
"It was well over six feet in height, with great branches of antlers growing out of its forehead. Nude, its flesh was a uniform ash-grey in colour. I appeared to be sexless, and it had grey, leathery wings extending far out behind it and joining with the night. It held a short, heavy sword of dark metal in its right hand, and there were runes carved all along the blade."
Strygalldwir, in 'The Guns of Avalon', Chapter Two
'"...Women," Ganelon said. "Pale furies out of some hell, lovely and cold. Armed and armoured. Long, light hair. Eyes like ice. Mounted on white, fire-breathing steeds that fed on human flesh, they came forth by night...'"
Ganelon on Hellmaids, in 'The Guns of Avalon', Chapter Four
"I beheld a naked rider on a hairless horse, both deathly pale, rushing in my direction. The rider brandished a bone-white blade; his eyes and the eyes of the horse both flashed red. ... His long white hair shed tiny sparkling motes, and when he turned his head I knew that he was coming for me, for I felt his gaze like a cold pressure across the front of my body."
Kwan, in 'The Hand of Oberon', Chapter VI
"A horned rider on a great black horse was bearing down upon me, something like firelight glowing in his eyes."
An un-named Chaosite, in 'The Courts of Chaos, Chapter 10
"He was waiting for me as I rounded a bend in the depression, a big, corpse-coloured guy with red hair and a horse to match. He wore coppery armour with greenish tracings, and he sat facing me, still as a statue."
Borel, in 'The Courts of Chaos, Chapter 10
These creatures of Chaos are not well adapted to conditions near Amber, and required the Black Road in order to function well, and indeed survive there. Chaos prisoners taken in the attack Corwin repulsed with the guns of Avalon soon withered and died without the Road.
Note that Chaosites burst into flames and burn away to nothingness when they die, regardless of how they are killed - it is not Pattern blades alone which do this. This is illustrated in 'The Guns Of Avalon':
"I slew them as fast as they approached, and they burned as they died. To my right, I saw Lance draw a similar line of Chaos, and he was muttering beneath his breath. Prayers for the dead, no doubt. To my left, Ganelon laid about him, and a wake of fires followed behind his horse's tail."
Corwin, in 'The Guns of Avalon', Chapter Three
However, being hit by a Pattern blade will cause a Chaosite to begin burning while they are alive, which will do them no good in the short or the long term...
From examination of the closed realm which was forced open by Fiona in 12 PPF the arts and architecture of Chaos (or at least of whoever lived in that realm) seem to be very stylised and baroque, with a vaguely oriental feel to them, using many reds and blacks. Sculpture and frescos only rarely portray humans, instead inhuman and semi-human things seem to be the main subjects. Some sculptures seem formed from bone. The other main art-form seems to be mosaics, generally hugely intricate, but abstract, portraying swirling geometric and fractal forms.
Apparently there are labyrinths under the main body of the Courts where the lowest of those who live in Chaos dwelled before the Amber invasion. If they even exist, they are almost certainly empty now.
Until 103 PPF Chaos was occupied (or at least surrounded) by the armies of Amber, under the command of Lord Shannon Feldane, and surrounded by garrison camps, both on the edge of the Abyss and in the surrounding shadows. However, with the destruction of the Citadel of Chaos over the Abyss in that year most of the forces were dispersed, leaving only a skeleton force on the edge of the Abyss.