Location from harry-potter-order-of-the-phoenix by
The hidden room on the seventh floor that becomes whatever you need it to be -- this year, it becomes the headquarters of Dumbledore's Army, complete with cushioned floors, practice dummies, and a mirror that shows Death Eater detection.
The Room of Requirement is where Dumbledore's Army trains, and it provides everything they need. Cushions so they can practice Stunning each other without injury. Books on defensive magic that Umbridge would confiscate from the library. A mirror that alerts them to approaching danger. Space enough for Reducto Curses, Shield Charms, and Patronuses. Harry teaches here -- really teaches, not just demonstrates but breaks down technique, encourages struggling students, pushes talented ones further. Neville improves faster than anyone. Luna practices with dreamy focus. The room becomes the most important classroom in Hogwarts, and the irony is that it doesn't officially exist. When Umbridge's Inquisitorial Squad discovers the DA, Draco Malfoy finds the room. The students scatter. The room is compromised. But what they learned inside it -- the spells, the confidence, the knowledge that they can fight -- goes with them.
For the DA, the room manifests as a spacious practice hall with cushioned floors, shelves of Defense Against the Dark Arts books, practice dummies, Dark-detector mirrors, and space enough for thirty students to practice spells simultaneously. Whistle on a peg by the door. The entrance appears as a door in a blank stretch of wall on the seventh floor, opposite the tapestry of Barnabas the Barmy teaching trolls ballet.
Also known as: the Room of Requirement, Room of Requirement, the Come and Go Room, the DA headquarters