Anonymous wrote:+1 on Wales.
We were there for a week this summer and toured around the south/central part in a rental car. There are a ton of castles, but our favorites were Caerphilly (they have an excellent scavenger hunt-type activity for kids) and Carreg Cennen (very remote, crumbling and surrounded by sheep). Links for both:
http://cadw.gov.wales/daysout/caerphilly-castle/?lang=en
https://en.wikipedia.org/wiki/Carreg_Cennen_Castle
We also visited Pembroke Castle, Larne Castle and Castle Coch, which were cool, but not our favorites. My husband went to see Cardiff Castle after we left, and he said that one was pretty good too. But seriously, the first two I mentioned were phenomenal.
I'm pretty sure Wales has the most castles per capita of any other country.