The Dragon Gate

The Dragon Gate is a fabled location at the top of the Shi no Yosai fortress of the Kamejin on the Kyoukai Shima border island, said to be the only prison in the world that can hold a Dragon. A giant gate made of an unmeltable metal, likely Fuccium, houses the five Dragons that lie within.

The Cage

Most of the Dragon Gate structure is made of the same unidentifiable black stone that the Shi no Yosai itself is made of, a stone that some say is a form of obsidian. While this would explain its near indestructable strength, some debate the possibility of obsidian being used in the construction. However, no better suggestions have been brought up.
At the front of this obsidian cage is a gate, made of an unmeltable metal. This gate is constructed with jagged metal, seemingly rusted over, yet still as sturdy as ever. It appears like the teeth of a dragon, making the cage itself the very maw that has swallowed up those Draconic figures it holds captive. The Dragon's Gate locks these dangerous creatures away from the world.
It has been said of the Gate's design:

"The only way to capture a Dragon is with another Dragon, and since man could not tame the Draconic forces... they built one to compensate."

Shi no Yosai

The Shi no Yosai, or Fortress of Death, is the central hub of the Kamejin military, both in land forces and naval forces. It is a towering fort that strikes fear into the Lizardfolk who settle nearby, as it is often where most military operations (especially ones against them) are launched from.

The Pit

Within the Dragon's Gate is the pit that houses the Dragons. It is a nest with a dirt floor and accomodations to make the Dragons feel at home. A mound of treasure was heaped up within to lure some in, and the Draconic visage of the gate does the rest of the work.
Some Dragons have laid eggs within the pit, leading to more young Dragons residing in there, though they are often not counted when considering who is in the Dragon's Gate. Some are let out, others killed before they can become too much of a danger because the Kamejin, while confident in the Dragon's Gate being able to withstand them, are not as confident they could take too many Dragons if something where to happen to the Dragon's Gate.
Still, those in charge of the Dragon's Gate say with confidence:

"Zome, Lord of Red and Reome the Wretched together could not destroy the Dragon's Gate, yet the Dragon's Gate could swallow them with ease."

Ryuukihei

Member of the Ryuukihei by Jarhed

There is a set of elite warriors gathered at the Shi no Yosai to guard the Dragon's Gate. These people are known as the Ryuukihei, but are also called the "Dragoons."
The Ryuukihei are a group of female warriors raised around various reptiles from birth, trained on how best to raise them and treat them with respect, while also knowing the best ways to subdue or kill them if necessary.
The Ryuukihei fight with a special style based on the blunt, powerful style of a Dragon. They use their blades like a Dragon uses their tails, while using their claws as a Dragon would. The Ryuukihei use special masks forged on the island of Ichisaki which can simulate the breath of a Dragon, where most Ryuukihei choose flame breath.
They are said to take care of the Dragons in the Dragon's Gate, though it depends on the individual Ryuukihei in how that manifests. Some actually care for the Dragons and treat them with kindness, while others abuse them or treat them coldly for the destructive deeds done by their brothers and sisters.

Origins of the Gate

It is said that when the Shi no Yosai was first built, its height and intimidating stature led to it becoming a perch for many Dragons. Some even started their nests at the top before being driven out by other Dragons. This became disruptive to the activities of the Kamejin military, as the Dragons would pick sailors off of their ships for food and would fall asleep blocking the exits or entrances to the fortress.
Therefore, an architect named Yuri Nabe came up with the perfect strategy to help the Kamejin deal with the Dragons. She provided blueprints for the Dragon's Gate and insisted the Kamejin government meet her exact specifications. In the meantime, she trained her skill with the blade and made sure the Dragons watched her do so.
When the Dragon's Gate was completed, it was placed beneath the Shi no Yosai, and the Wind blade of the Ken no Yoso was handed off to the military. In the meantime, Yuri Nabe went to the top of the Shi no Yosai and called forth a challenge to the nearby Dragons. She claimed that she could defeat each and every one of them in a fight if they landed on the top of the fortress. The Dragons, not believing her, landed to fight.
She preoccupied them with her combat, meanwhile the wind blade flew the cage up and onto the Shi no Yosai, trapping the Dragons and Yuri Nabe atop the fortress. The Dragons mocked her for dooming herself, but all she said was:

"I know I'll die and leave this cage soon enough, but Dragons live for thousands of years. You'll live forever with the knowledge that even when I die, when my bones are all that remains, I will have beaten each and every one of you. Besides, I'm not trapped in here with you... you're trapped in here with me."

— Yuri Nabe

Nabe went on to actually manage to kill one of the Dragons in the cage before she succumbed to her wounds and was unable to fight. She then ran her own blade through her stomach so that the Dragons would not get the satisfaction of killing her.
The Ryuukihei were modelled after Yuri Nabe, which is why they are an all-female force of warriors. The Kamejin believe that, like Yuri, other women have better odds against the forces of the Dragons, though they have no particular reasoning beyond what Yuri Nabe was able to accomplish.