If one follows the narrow stone staircase leading from the Entrance Hall to the dungeons, the entrance to Snape's office is halfway down the cold passageway leading from the foot of that staircase. Snape's office is adjacent to the Potions classroom, or at least close enough that someone could slip from one to the other without being spotted during a class.
The walls, which are in shadow, are lined with glass jars, including the wall behind Snape's desk. Each jar contains some potion (differently colored jars) in which suspended slimy bits of some animal or plant (different ones in different jars).
