Ken, What and where is that? And what's with all the scratches all over the place?
The figure in the middle seems to be a simple construction according to the Ad Triangulum or Daisy Wheel mode. The outer circle's radius is equal to the diameter of the inner circle, and two arcs of equal size are scribed on it marking 4 of the six points of the daisy wheel. All the scratches make it difficult to make out.

What is this thing? I have no Idea. Maybe it's the work of some geometer musing with his compass, maybe it's a geometric symbol set into the building for spiritual reasons.

Ever think that the use of the daisy wheel could simultaneously have both of its theoretical applications? Even in a single instance?
The daisy wheel was a tool or at least a form of a tool for geometric design, because without the circle you really can't do anything else. At the same time it was a symbol of protection, harmony, or some type of divine blessing. How so? Because to the Medieval mind, geometry was sacred. Geometry was a reflection of the divine, and of the order of Heaven, particularly since Heaven was thought to be circular in nature, with its center at the Godhead.
The Ad Quadratum reflected the sphere of the earth, with the square pointing off to the four corners of the world.
The Ad Triangulum reflected the elemental essence of creation, with the two triangles overlapping to reflect the full alchemical cycle. Plato is said to have had above the door to his academy a sign that read "let no man that is ignorant in the ways of geometry enter here." or some such. Geometry was the foundation of the ancient world and its philosophy, so why would geometric symbols not hold sacred value to these people?

Don't discount the divine...