The Slytherin Dungeon, also known as Slytherin Common Room, is located behind a wall in the cellars of Hogwarts. To get into the common room you must first know the password to the room (it changes every fortnight) and then you must know which wall to whisper this password to.

If you don't know either, then you'll just be left standing there, literally talking to a wall. If you're not a lost Slytherin then I wouldn't bother trying to talk to the walls around here; they're quite damp from the humidity of the lake above and sometimes they make noises....

You can hang around the hidden entrance to the common room here (especially if you're waiting on a friend) but be careful... snake pits aren't pleasant places for outsiders.