Although smaller, Caldicot Castle reminded me somewhat of Warwick Castle in that it was so well preserved and most recently used as a residence. The remnants of the surrounding moat are still apparent.
The castle sits in a 50 acre park so bring a picnic lunch and spend the afternoon.
The Normans built a structure here as early as 1086 and the current structure dates from 1221.
The wikipedia article is here. Their web site is here. There's another good web site here.