The Dark Arts Tower comes in handy
Fairly early into Hogwarts Legacy, the player character will gain access to a semi-secret area called the “Undercroft.” This is the main base of operations for quests dealing with your classmate Sebastian, and you probably want to know how to get back there quickly. Here’s how to do that!
After you unlock it, you can access it from the Defense Against the Dark Arts Classroom Floo Flame spot
You’ll unlock the area after finishing the mission In the Shadow of the Undercroft. Once that’s done, you can return to it: which you’ll need to do several times throughout Sebastian’s side story, and the main questline.
To get there, select the Defense Against the Dark Arts Clasroom Floo Flame fast travel location. Turn to the right and go down the stairs. Turn right and go down the hallway that looks like a dead end. Enter the clock by interacting with it, and you’ll be in the Undercroft.
You don’t need to speak a password or do anything special (or cast anything) when interacting with the clock. Just walk up to it, and as long as you meet the aforementioned perquisite questline, you’ll get in.