Rebma is located roughly 200 feet below the sea near to Castle Amber. It is accessed through a stairway down through the ocean known as the Faiella-bionin, marked at the top on the beach by an eight-foot-high cairn of grey stones. The steps are shallow, with a step only every 5 feet or so. The Faiella-bionin is lit by ghostly lights, one every fifteen steps starting some 50 feet below the surface, alternately on the left or right of the stairway. There are roughly fifteen of the lights altogether. (Corwin passed eleven lights before he paused some 200 feet from the gates, went twenty steps more and turned to fight 100 feet from the gate.)
When we entered the vicinity of the thing, the waters grew warmer and the stairway itself became clear: it was white, shot through with pink and green, and resembled marble but was not slippery despite the water. It was perhaps fifty fee in width, and there was a wide bannister of the same substance on either side.
We entered the vicinity of the first light... It appeared to be a flame, about two feet in height, dancing there, as atop a huge torch.
At the base of the steps is a decorative archway that shines like alabaster and is carved with Tritons, sea nymphs, mermaids, and dolphins. It is normally guarded by a sizeable band of guards. From the archway, there is a walk of 15 to 30 minutes to the main gates of the city. The city itself is described as:
The Golden gates of Rebma stood before us. We passed through them. We entered the city.
Everything was to be seen through a green haze. There were buildings, all of them fragile and most of them high, grouped in patterns and standing in colours that that entered my eyes and tore through my mind, seeking after remembrance.
...Inside the city, we were conducted up a wide avenue, lighted by pillar flames set at even closer intervals than on Faiella-bionin, the people stared out at us from behind octagonal tinted windows, and bright-bellied fishes swam by. There came a cool current, like a breeze, as we turned a corner; and after a few steps, a warm one, like a wind."
"We were taken to the palace in the center of the city, and I knew it as my hand knew th glove in my belt. It was an image of the palace of Amber, obscured only by the green and confused by the many strangely placed mirrors which had been set within its walls, inside and out. A woman sat upon the throne in a glassite room I almost recalled..."
As in Amber, deep beneath the palace is an enormous cavern and network of tunnels, within which is the pattern room. Corwin began to recall a bit as he reaches the Pattern,
"It was a huge gray door of some slate-like substance, bound in metal, towering to twice my height. I remembered something about the size of Tritons as I regarded that doorway."
Note that this implies that Tritons are real within Rebma, although he hadn't seen any on his visit until that point. They are evidently larger than humans, but nothing else is said of them.
Those on the Faiella-bionin and within the city can breath as if in air, and stay weighted without normal bouyancy, but still react in many ways as if underwater. Bubbles form around their heads, and their voices have a ringing sound. Fishes can still swim within the enchanted water, and may swim past breating people. There is no feeling of increasing pressure, but they do feel the wetness of the water. They can walk normally with a semblance of weight, but do not fall normally. So one can step off a stairway and swim down rather than walk.
The realm of Rebma is ruled by Queen Moire, but little else is known about the rule of that realm. There is mention of Moire's daughter Morganth, but none of Morganth's father or Moire's husbands. Moire bore no love for King Eric, and showed no fear of his wrath. It appears that he had no means to attack Rebma. She rejected lending troops to assault Amber, but not because of the difficulty of it but rather because strife in Amber would supposedly cause reflected problems in her own kingdom. So Rebma at the start of the Patternfall War was wholly independent and had considerable troops.
The queen herself rules from the central palace, which was on top of a hill like Castle Amber -- with the Pattern in the depths beneath. Queen Moire is described as follows:
"A woman sat upon the throne in the glassite room I almost recalled, and her hair was green, though streaked with silver, and her eyes were round as moons of jade and her brows rose like the wings of olive gulls. Her mouth was small, her chin was small; her cheeks were high and wide and rounded. A circlet of white gold crossed her brow and there was a crystal necklace about her neck. At its tip there flashed a sapphire between her sweet bare breasts, whose nipples were also a pale green. She wore scaled trunks of blue and a silver belt, and she held a scepter of pink coral in her right hand and had a ring upon every finger, and each ring had a stone of a different blue within it."
She also has an affair with Corwin quite freely, which may suggest something about Rebman romantic mores.
Canonically, the history of Rebma is not well documented. We know that at some distant point in the past, the Queen of Rebma was Moins. Oberon fathered a Llewella by Moins while he was married to Clarissa -- mother of Fiona and Bleys. He later divorced Clarissa (after which she bore Brand), but never married Moins -- but he recognized Llewella as legitimate, supposedly to spite Clarissa. .
Queen Moins had another daughter, Moire, who eventually became queen. Moire in turn had a daughter, Morganthe. As Moire described it later to Corwin and Deirdre:
"Surely you recall," she said, "that one time Prince Random came into my realm as a friend, and did thereafter depart in haste with my daughter Morganthe."
"I have heard this said, Lady Moire, but I am not aware of the truth or the baseness of the tale."
"It is true," said Moire, "and a month thereafter was she returned to me. Her suicide came some months after the birth of her son Martin. What have you to say to that, Prince Random?"
She then sets out to judge and punish Random. The details of Morganthe's suicide are not mentioned, but Moire appears to hold Random responsible. Intriguingly, she set as the punishment as follows:
"Therefore, I will punish thee . . . You shall marry the woman of my choice and remain with her in my realm for a year's time, or you will forfeit your life."
It is a rather unusual punishment, to say the least, as an alternative to death. Further, she suggests that the marriage would be to Vialle's benefit within the Rebman social scene -- even though she apparently assumes that Random will leave her at the end of the year. There are many open questions here. Why does she feel that Vialle should benefit, and what social respect comes from having been married to a Prince of Amber for a year.