Evidently there are something like 300 medieval castles that have substantial stonework remaining in the UK. Don't know about Ireland though. There are over twice that number if you include earthworks only - plus all the rest that are earlier.
I have been to nine listed on this map, and at least 20 others that aren't.
They should include all the Roman forts marked on the OS map, and when you go there there’s only the slightest hint of rectangular mounds in the field. Also, it should be “British, Yorkshire and Irish Islands”, if we’re going to keeping expanding it based on levels of beef.