Dungeons and Sub-Levels

Beneath the ground floor of the castle are found many passages, dungeons, and chambers. Some of these are easily accessible from doors that open off the Entrance Hall. Others are hidden and can only be entered by magical means.

dungeons
Snape's Office
Potions classroom
Slytherin common room
kitchens
Hufflepuff common room
The Chamber of Secrets
The chambers of the Philosopher's Stone

dungeons
The dungeon passages and rooms under the castle have stone walls and are lit by torches. Nearly Headless Nick held his Deathday Party in one of the "roomier" dungeons (CS8).

Potions classroom
Potions classes are held in a dungeon classroom, large enough to hold at last twenty students, their cauldrons, and their work tables. The classroom is quite cold, especially in the winter, when students can see their breath. There are pickled animals in glass jars along the walls (PS8). There is a basin in the corner of the classroom into which icy cold water pours from a gargoyle's mouth. A student supply cupboard is in the room, along with a blackboard on which the instructor can write directions for student work (OP12).

Slytherin common room
The Slytherin common room is a long, low-ceilinged, dungeon-like room filled with skulls and the like. There are high-backed chairs and a carved mantlepiece on the fireplace. The walls are rough stone. The light is green from the green hanging lamps and because it's located under the lake (CS12, DH23).

kitchens

The Hogwarts kitchens are located directly below the Great Hall and are just the same size. They have high ceilings and a great brick fireplace at one end. Mounds of brass pots and pans are heaped around the walls. There are preparation tables directly below the four house tables in the Hall above, and when the time comes for the food to be served, it is magically transported through the ceiling of the kitchen onto the plates. The kitchens are staffed by over a hundred house-elves. To get to the kitchens, take the door to the right of the main staircase in the entrance hall. Follow the corridor until you come to a painting of a bowl of fruit. Tickle the pear and it giggles and becomes a door handle (GF21).

Hufflepuff common room
Accessed through a still life painting found near the kitchens; to reach it, you enter a door to the right of the main staircase in the Entrance Hall and down a staircase there (GF17). Although it is therefore at dungeon level, it is "as dissimilar as possible" from the Potions classroom. As we have seen in the other common rooms, it is decorated in House colours, which in this case means that there are lots of yellow hangings; like Gryffindor Tower, it has a lot of comfortable armchairs. There are "little underground tunnels leading to the dormitories, all of which have perfectly round doors, like barrel tops" (BLC, JKR).

Possible plan of the kitchens and the dungeons

These maps takes the details from the books into account as well as the standard structure and architecture of castles. While not canon, this map is probably as close to the "real thing" as we're ever going to get.