Demon King Castle (魔王城) was the base of operations of the Demon Army during Satan Jacob's campaign to conquer Ente Isla. It was here that the the Demon King lived, and where he was defeated by the Hero Emilia, resulting in his self-imposed exile to Earth.
Located somewhere in the middle of the Central Continent, Demon King Castle was the home of King Satan and the Demon Army's seat of power in Ente Isla. The building was large and foreboding, consisting of several pointed towers and turrets, the windows glowing with a mysterious eerie light. The interior of the building contains several winding hallways, and in the heart of the castle is the dark citadel where the Ruler of Demons sits upon his throne. In a small room behind the throne is the place where Satan planted a purple crystal he had received from the Angel Laylah, though unbeknownst to him, the stone was actually a fragment of Yesod split from the fruit of the Tree of Life.
Following the Demon King's defeat, people attempted without success to destroy the evil residence. The building was then surrounded by walls to prevent anyone from approaching, less they be harmed by the remaining magic.
During his attempted conquest of Ente Isla, King Satan built the castle on the Central Continent in order to establish a seat of power from which to command his armies. He dwelt in the dark castle for the majority of the invasion, and it was here that Alciel fled to upon the Demon General's retreat from the Eastern Continent. Once the other Archdemons had fallen to Knight of the Church, Emilia Justina, the resistance continued on to the Central Continent in order to confront the leader of the Demon Army. The Allied Knights Order then stormed the castle, the final attack led by Emilia, Albert Ende, Emeralda Etuva, and the High Priest Olba Meyer. The four heroes broke through the castle's defense and confronted King Satan and Alciel in the throne room. Emilia fought and defeated the Demon Ruler in a ferocious battle, slicing off one of his horns and forcing the demons to flee to another world through a portal in the sky.
After the defeat of the Demon Army, the castle remained unoccupied for some time. During this duration, people attempted to deconstruct the evil dwelling, but were always repelled or contaminated by the dark magic that remained. Unable to dismantle Demon King Castle, the human occupants of Ente Isla simply chose to build a wall around it in order to prevent others from going near it. The Angel Laylah later enters the castle to retrieve the fragment of Yesod planted by King Satan, sending it to Earth through the Gate in the form of a large fruit. The mysterious object later take the form of a small child named Alas Ramus, who is then adopted by Sadao Maou and Emi Yusa.