The abandoned tower in the castle's east wing had been completely cleared out. Its three spiraling stone stairways had been converted into a vertically connected practical training space. The floor was covered with a dark brown, leather-like flexible mat, while adjustable Illusion Generators had been embedded into the walls—alchemical devices capable of projecting virtual environments such as swamps, forests, and alleyways.
A silver Magic Synchronization Core had been installed at the top of the tower. Using dozens of dragon heartstrings as its primary control medium, it powered every adjustable danger-level switch in the grounds, ensuring that the emergency teleportation devices could respond instantly under any circumstances.
A Ravenclaw sixth-year prefect had been assigned to test it first. He stood at the entrance in a daze for a long while before entering. His gaze lingered on the plaques of the slowly rotating Illusion Generators, where a line of tiny lettering had been engraved at the bottom: "...Constructed by Gringotts Fourth Alchemy Workshop." He made no comment on those words, merely stepped into the center of the grounds and tightened his grip on his wand.
The first class held in the Defense Against the Dark Arts Practice Grounds was led by Severus Snape.
He stood beside the control console at the edge of the grounds, the hem of his black robes draped across its metal baseplate. One hand rested beside the danger-level dial without touching it. His eyes were fixed on the students in the center, enveloped by an illusory swamp so realistic that even each drifting wisp of mist had its own temperature difference.
As a Gryffindor third-year girl crossed a patch of "swamp ground," the surface suddenly collapsed beneath her. She lost her balance in an instant, but before she could fall, the teleportation band on her left wrist activated