Hogwarts walls tend to allow more coming and going than non-magical walls. To make the Room of Requirement appear, a person has to walk past the section of blank wall three times concentrating hard on what is needed, for instance, and when the gargoyle guarding the head’s office is given the proper password, it will leap aside and the wall behind it will split in two to reveal a spiral stone staircase that moves like an escalator (CS12, GF30, OP22). On the other hand, some apparent doors in Hogwarts were really “solid walls just pretending” (PS8).
The walls in the Syltherin common room (CS12) and the walls of the Hogwarts kitchens (GF21) are both described as made of stone.
Harry was able to hear the basilisk through the castle walls as it spoke parseltoungue and slithered through the pipes (CS16).
The walls within the Room of Requirement can apparently change, as well. The room as it appeared when used as a hideout for the D.A. during the school year of 1997/98 was described as looking like a ship cabin in part because of wood paneling on the walls (DH29).
Hogwarts faculty have been known to line or decorate their office walls with items that reflects their interests or priorities - possibly with the help of fixing charms.
- Professor Lockhart covered his office walls with pictures of himself (CS7)
- Professor Snape used shelving along his office walls to store potions in which were suspended slimy bits of animals and plants (OP26)
- Professor Umbridge adorned her office walls with colorful ornamental plates depicting kittens (OP28)