Also see Armour and Suits of Armour
Suits of armour stand in a number of places around the castle. They are empty, mere metal and leather, but they do seem to have some amount of magical animation and even sentience to them.
They seem to move about: The people in the portraits kept going to visit each other, and Harry was sure the coats of armour could walk. (PS8)
The suits of armour creak on their own (CS5, DH29). There is a suit of armour near the kitchens (PS12) and a suit of armour stands next to the front doors of the castle (CS12). They still moved on their own, but did so silently after they’d been oiled for the visit by Durmstrang and Beauxbatons (GF15)
A suit of armour stands in the corridor near the office of the Defense Against the Dark Arts teacher. It turned its head to watch Harry as he hid behind it (OP29).
In the Battle of Hogwarts, McGonagall uses the spell Piertotum Locomotor to call the suits of armour and statues of the castle to animate and defend Hogwarts (DH30).