Question! Even though "i" is said to translate as "in, on, around", is it more correct to use "på" when you mean "on"?
At the wall means "a location near or next to" a particular wall, e.g., "The family was at the wall when they were arrested."
On the wall means "in direct contact" with the surface of it, e.g., A spider walks on the wall. Pictures hang on the wall. The graffiti artist is painting on the wall. He was leaning on the wall, waiting for his friends.