The magical barrier known as the Wall has long kept the empire at bay, and thereby maintained the peace, yet not entirely without struggle. Sustaining the Wall imposes an immense physical burden upon the king, but to let it fall for even a moment would be to invite total annihilation.
The Wall (魔法障壁, Mahō shōheki?, lit. Magic barrier) is a transparent magic barrier that surrounds the city of Insomnia in the Final Fantasy XV Universe. Erected 150 years before the events of Final Fantasy XV, it protects the Crystal within from outsiders and is maintained by the ruler of Lucis, who uses the Ring of the Lucii to channel the Crystal's power. Upholding the Wall drains this ruler's life-force.
Over 150 years ago Niflheim Empire declared war on Lucis and Accordo. Accordo was annexed, and the king of Lucis erected a magic barrier to protect his country and take up a siege position. Originally the barrier covered the continent, but thirty years ago, King Regis's father, King Mors Lucis Caelum, consolidated the power of the barrier over Insomnia—strengthening it—to protect it from the empire's magitek soldiers. Insomnia thus became isolated from the rest of Lucis, and Niflheim began to make inroads into Leide, Duscae and Cleigne regions.
On the day a peace treaty is to be signed between Lucis and Niflheim, magitek infantrymen breach the Wall and infiltrate the Crystal Chamber. They bring the Wall down, and Niflheim airships invade the Crown City. The absence of the Wall allows daemons to spawn in Insomnia plunging it into chaos.
King Regis and the Kingsglaive use protect magic to erect magical walls to protect themselves and those with them, which is similar to the Wall but in a much smaller scale. After Regis dies, the Kingsglaive members lose the ability to use magic, but Nyx Ulric again projects a barrier to protect Lunafreya Nox Fleuret when he gains the power of the Ring of the Lucii.