One minor thing I caught the other night was that Cadfael's workbench is at least 400 years ahead of its time. It has a tail vise on one end that I've not seen illustrated or in artifacts prior to 1600. It's a joiner's bench they probably thought looked vaguely medieval because it was old and worn. We've also caught some anachronistic architecture, windows, etc. The historical books she wrote under her own name are even better, IMHO, than the Cadfael series, and I enjoy the Cafaels a great deal.
Quartet tells of the last kings of Wales.