Beaumaris Castle is visually near perfect, concentric in design and with a full moat crossed by a drawbridge. It is claimed to be technically the best in the UK.
Unless you want to hear about the sieges and conflicts you won't be disappointed - but it does seem to have been rather a peaceful building!
There's no doubt this made it a success story since it seems to have fulfilled Edward I's purpose of holding down the Welsh without any real military activity.