Its walking down. See the little ledge type thing at the end of each step? If it was walking up, then when you walk down you could hit your toe on it, and well itd be poor craftsman ship. But the camera is facing up, so you're seeing underneath the stair ledge. Also the shadowing would be wrong if it was walking up
Check where the cat is stepping. If the cat is going down, the cat would be stepping on this side of the stairs -->|__ which is impossible. The cat is going up