Dover Castle was one of the important Royal Castles during the Norman period and has remained an important part of England (and, later, Britain’s) defences against foreign invasion … yes, even during WW1 and WW2.
Continually expanded and modernised during the period, the core medieval castle remains much as it was externally, though the interior has had ‘modern’ (mostly 18th and 19th century) buildings added. The central Keep, however, is very much ‘as was.’ Some of the internal rooms have been furnished and decorated to show what they would have looked like during the medieval royal heyday.
Also on the grounds of the castle is the parish church of St. Mary in Castro which incorporates the remains of the famous Roman Pharos Lighthouse that watched over the Roman Port of Dubris.
There are views of the modern Port, more or less the same site (but improved) as the Roman Dubris, from the WW1/WW2 era artillery spotter’s control bunkers which are the seaward edge of the walled area – on the cliff’s edge looking over the channel. The spotters here gave control orders to gun positions up and down the coast.
I only went to Canterbury to see the Cathedral – didn’t have time for anything else. It’s a Cathedral. Lots of religious-y stuff. Stained glass windows etc.