Dimensional Gateways

Dimensional Gateways (次元のゲートウェイ Jigen no Gētouei) are special gates crafted by the priests of the goddess Amara in the Empire of Illa, roughly three thousand years prior to the beginning of the author's storylines. In resume, the Gateways are artifical wormholes created using magic mixed with divine energy

Overview
The Dimensional Gateways are, in a short resume, wormholes. This is, a shortcut through the continuum of time and space, allowing someone to travel quickly between two points of space

As Amara couldn't be summoned to Earth Land due to her position as the Keeper of the Far Keep, her priests, the main religious power of the Empire of Illa at the time, channeled her divine power in order to construct artificial wormholes in the form of several gateways. These gates would allow the priests to access the Far Keep and meet their goddess personally. Also, this would allow them to access any place in the universe to to the Far Keep's position as the center of the known universe. By channeling the arcane energies present there, the mages and priests could grew stronger than normal humans

After the fall of the Empire, the priests, in order to prevent Zentopia from finding the gateways, burned all their maps and sealed them. By sealing them, it was determined that only if the blood of the members from the bloodlines of the priests could re-open the Gateways, and, once reopened, they could be used only once before destroying themselves automatically. Done this, Amara locked the Gateways from the Far Keep's side

Two centuries before The Fires of Heaven, Andraea, hunted by the knights of Fiore, saw one of the priests' descendants being killed in the front of a Gateway, and it opened, swallowing the knights and her. Upon entering the Far Keep, she met Amara herself, and, after talking with the goddess, she give her a reason to live. Amara departed, and the Gateway destroyed itself soon afterwards

Intro arc
Upon her death, Andraea is taken to the Far Keep and meets Amara again. She remembers when she crossed the Gateway and met her the first time

Trivia

 * They were based in the Doors to the Abyss from Pandora Hearts