Royal One (also known as The Demon King's Classroom) is a location within Babylus Demon School.
This prestigious classroom was built solely for the sake of the previous Demon King. Since those days when the Demon King attended, the classroom has been sealed off, entry forbidden by all to this priceless relic of the school.
The door to Royal One is sealed by at least four locks and several chains entwined with them. A rope cordon is set in front of the door.
The classroom's full details are never revealed, but it has at least two stories, with high ceilings, luxurious carpets on the floor, priceless vases as decoration, and a spacious lecture hall with a blackboard. The classroom is implied to have special facilities such as a napping room and a gymnasium. The Demon King's Autokeep Magic permeates the classroom, keeping almost everything in perfect condition, even after being sealed away for years.
One particular centerpiece of Royal One is the ornate throne across from the blackboard. While it has undoubtedly seen better days, covered in cracks, breaks, and smudges, it still retains an air of dignity, such that when Iruma sat on it, the class was rendered speechless, as if the Demon King himself were sitting on the throne.