It’s missing the extensive Napoleonic-era Western Fortifications on the opposite side of Dover from the much more famous Dover Castle.

https://en.wikipedia.org/wiki/Dover_Western_Heights

But to be fair they’re much less well known, and quite well concealed, and are partly on private land, with big fences, cameras and scary warning signs. Tourists don’t seem to be much interested in post-medieval fortifications.

It's probably somewhat regional, as the ones in Canada seem to have pretty good tourism interest, especially when they're right inside a city like Quebec and Halifax.

But we're also very starved for true "Medieval Castles" like Europe or east Asia, so we take what we can get!