About Mythrin and the Urdar
The dragons call their world Mythrin, which in their language means "our world." The old Earth, their original home, they call Ardrin, which means "demon world." The word for humans (ardin/ardini) is the same as the word for demons.
They call themselves (their race) the Urdar, which means "the wise." This refers to their telepathic abilities, which enable them to (in their own estimation) know more than whoever they are confronting. They are also shape-shifters, able to take any form, although their dominant form is draconian. With all these gifts, dragons have a vast superiority complex. They see themselves as the crown of creation.
Amelia calls Mythrin the Ruby Kingdom; the dragons do not use the word kingdom, because they have no kings. The nearest thing is chiefs, who are first among equals and do not inherit the rule, though they may often be related to the previous chief. Instead, they are chosen for their qualities of leadership, dominance, courage, nobility, intelligence, and wisdom, and are usually acclaimed, after various elders and leaders have spoken in conclave, sometimes also after defeating a rival in combat.
Their city is Sissarion, which means "Gathering Place." About a quarter of the Urdar live there. Dragons prefer wide-open spaces and are not herd animals. Many live apart, singly or in small family or tribal groups.
The Urdar know that the region they inhabit--which is semi-tropical and mountainous--is only a small part of their world. They have scouted far enough to know there are no other inhabitants on their continent, but are not sure what may lie on the other side of the globe--on the far side of the ocean. They try to explore as much as they can, not for curiosity but simply because if there are any enemies around, they want to know. But the oceans have stopped them. Dragons can fly very far, but not forever: those who tried to cross the ocean have never returned. It is possible the rest of the world is all ocean.
The dragons are few in number on Mythrin and they are not technological by nature. As a result, Mythrin appears to be largely wilderness, even after some 1,000 years (that is, 10,000 Mythrin years) of dragon occupation.
Time on Mythrin
In Mara's universe, time passes at a rate roughly 10 times faster than in Amelia's universe. (Ike and Simon work out that the difference is actually closer to 9.5 times.) So the time that elapses between the first and second books of the Passage to Mythrin series is six months for Amelia, but 57 months (four years and nine months) for Mara.
Names and words are important; not tools
Personal names, such as Marathynarridin, are long because they contain descriptions of the individual and references to ancestors. The Urdar do not use nicknames or shorten their names. (Mara makes an exception for Amelia, but she permits no dragons to use her shortened name: that would be grossly insulting.) Because they generally use mind-speak among themselves, they usually do not speak their names aloud, although they will hold them in mind
Dragons cannot lie outright, but they can conceal knowledge and raise mental barriers. Since they are primarily mind-speakers, it follows that their mental processes are subtle and complicated. They like to speak in riddles, a sort of verbal sparring. To humans, they appear treacherous and untrustworthy. But to almost all dragons their sworn word is unbreakable, which is why they will go to great lengths to avoid giving it.
Urdar culture is handed down orally and telepathically. They tell tales and sing sagas, but they do not write.
The dragons pass from waking to dreams with little sense of difference. Dreams are another realm of life, and in dreams reality is most true and most real. They spend a lot of time exploring the meaning of dreams.
Because Mara and Amelia have once shared dreams and have become mentally linked, in great need Mara is able to reach out to Amelia in a dream through the dimensions, and even to pass an object--the ring of the Urdar.
About the gates:
Mythrin was deserted when the dragons arrived, roughly 1,000-1,200 Earth years ago. The earliest inhabitants were probably human or human-like, judging by the ruins of their cities and artifacts. Unlike the Urdar, they were a building and exploring people, and had a high post-technological civilization that incorporated elements of what seem like magic to Amelia, but may simply be highly advanced science -- in particular, the system of interworld gates and passages that still operates, long after the makers vanished. (The dragons have no idea what became of the gate-makers, and they don't care, but Simon and Ike are curious.)
The Urdar were always sensitive to the vibes put out by these gates and knew where they were. They have not explored all of their own world, although some young ones have dared to pass some of the dimensional gates: not to seek knowledge but to demonstrate their courage. However, they have explored enough to know that Mythrin is a dimensional hub--one of the places in the cosmos where many worlds and universes come close together and sometimes overlap.
The gates make it possible to pass safely from one world to another: the "tunnel" or passage between is the transition place, where energy differences between the worlds are equalized--something like an AC/DC converter or a power transformer. To travel directly from one world to another could be fatal. That's why the places where the membrane between worlds is very thin, and where pressure from the other side forces cracking or tearing, are highly dangerous--the more so since such cracks usually only appear when there is a great difference between the energy levels of two worlds. It is not necessarily evil or "dark" energy, but it may be perceived as such because the emanations can have unexpected and sometimes harmful effects on people in the receiving world.
One interesting effect is what Mara calls "like breeds like." The surroundings of a gate on one world usually resemble the surroundings on the other world. Ike speculates that the gates themselves cause this to happen, and that it may be a way of easing the passage between worlds.
There are three ways a gate will open. The gates can be opened by one who has studied them and has some understanding of them: as Ike guesses, it's a quantum thing, a folded-space construction. To open a gate requires enough mental energy to manipulate the energy of the gate in a certain way, much like a hand turning a key in a lock--only it's much more complicated than that. The seekers of the Casseri, the human refugees who fled to Mythrin in The Prism Blade, have mastered these mental skills over millennia.
To dragons, with their highly developed and finely tuned psychic powers, the gates open at a touch. This is how Mara came through the gate to Earth in The Ruby Kingdom.
The third way to open a gate is to touch it with something (or someone) that comes from the target world. This is why Mara's ring will open the gate leading to Mythrin for Amelia, and why either Simon or Amelia can open the gate to Earth just by touching it. When Pier comes through to Earth with Simon, she is careful to bring with her a stone from Mythrin.
As Ike guesses, Earth is another hub world. He wonders what other gates exist on Earth, leading to what worlds, and if the presence of these gates explains a lot of ancient beliefs.
The gates can't be destroyed: at least. not by conventional means. They can be sealed by someone who knows how (like the Casseri seekers) but that takes an intimate knowledge of the particular gate and one way won't work for all.
The local housing or "marker" can, however, be destroyed or removed, making it impossible to reach the gate. In keeping with like-breeds-like, when Simon and Amelia try to get at the gate in the gorge, they find the cave is blocked by a rock fall, said to have been caused by water freezing and thawing in the spring. But oddly, the rocks look scorched. On the Mythrin side, the museum and mountainside were destroyed by dragon fire.