Possibly a telescope, or else just the edge of a flattish piece of rock which just happens to look that shape from this angle.

Directly in front of it (to the left of the picture) is a white patch (immediately below the head of your red arrow) which I'd noticed and wondered if it was the top surface of another wall as it seems consistent with the colouration and flatness of the walls in the right of the picture. I can 'see' that this wall curves around your 'telescope' as I'd expect it to, but again, it is probably just different patches of rock that present a certain impression to the viewer. (Still surprised nobody has identified the 'man' I'd spotted in reply #52

)