I think Crayons has the right of it....At this scale most of all. only draw the stones if you would be able to see them in scale, and at that only a few at a time with those at the edge of the patch being incomplete. At the scale you are at here, you would only see the variation of stones and major repairs so cracks would be sufficient I think. I would'n put too many of them either unless you were gonna grunge things up later. just my own humble opinion, that......
Looks good though. Not too sure on what scale we are looking at here after you put those buildings in. Looks like several of them would fit in the gatehouse.....By the way, how long to do the initial "doodle"?