The phrase "on a low wall" is correct and usable in written English.
You can use this phrase when referring to an object or a person that is situated on top of a low wall. For example: "The children were sitting on a low wall eating their lunch."
Behind a low gate inside the Pembury complex, around eight 20-something men were sitting in a huddle in a children's play area around 3pm, some sitting on a low wall, one or two perched on swings or rocking toys, talking earnestly and quietly among themselves.
A short walk from the one-stop camp, four Syrians sat on a low wall overlooking a side street.
A car is suspended diagonally across the sidewalk of one of the main roads, its rear impaled on a low wall.
There she sits, an artful stack of driftwood to one side of her bench, a neat row of plants behind her on a low wall and a gray horizon beyond.
We sit on a low wall opposite and chat quietly.
We sat on a low wall to talk, but, with the speakers blaring, Daza was virtually inaudible.
The collection of trailers sits on a two-acre plot on one side of a low wall, a humble contrast to the substantial, well-landscaped dwellings on the other side.
The older man had also stopped reporters from talking to the local youths who climbed on top of a low wall surrounding the compound and sat there for hours, staring at the journalists inside.
Painted on the side of a low wall that circles the area are five words in large, childlike lettering.
In a second experiment, the researchers tested the wolf pups to determine whether they would follow the gaze of a person as she stood staring at a spot on the other side of a low wall.
He hops off a low wall on our way around the city gardens where we meet, and scurries up all four floors to his flat, clutching his shopping bags.
