The Room of Requirement. What else is there to say about this room? Not many students know about its location and it is unplottable, so therefore it's not on the Marauder's Map. Though, students whose parents were part of Dumbledore's Army are told about the Room of Requirement, but not all of them know of its location or that it's even on the seventh floor. But for students fortunate enough to know of its location, a world of wonder awaits them. A world where anything is possible is at their fingertips.
