You probably did this before, but in case not, check some refs of cloisters:
http://images.google.fr/images?hl=fr&source=hp&q=cloitre&btnG=Recherche+d%27images&gbv=1Some have strong direct sunlight, some not (either because of weather, north-facing side, night).
What Ben2theEdge calls 'ambient light' is
diffuse light, with 2 possible sources: the sky + reflected light from the ground and walls.
They can be noticeably distinct, mainly by their color as can be seen in some images.
I agree with CrazyMLC about the outlines, though it may depend on your bg.
The arches look weirdly irregular, in a not too realistic way (esp since the columns are very straight).
http://en.wikipedia.org/wiki/ArchThe keystones look like they recess into the arch (probably a by-product of your circle line): this never happens, they should either be level or project inside.
The stones and joints look much better on the columns than arches.
The paving repeats with a too short interval (at least some elements), and it looks sideways. Maybe this is intentional, but it doesn't fit with the arches.