My local is Dover Castle - it's interesting just because of the sheer range of eras the different structures cover. The first verifiable structures were built by the Romans, of which a lighthouse still survives. It's perched on a cliff top that overlooks the harbour, and has historically been used to keep dirty sea-invaders from raiding Kentish orchards. This didn't work out so well in the 1066 Norman invasion, back when it was just a clay rampart.
The defenders surrendered without a fight, following which the Norman soldiers accidentally burned the town down. I would be sceptical as to how you could accidentally set a town on fire, but William's guards also set fire to the cathedral he was being coronated in while the ceremony was ongoing because they were spooked by the sound of the English nobles cheering. Fire's just their thing.
The keep and concentric walls are from the 12th century onwards, with further additions and additional casements being constructed during the Napoleonic period. Interestingly, this is also when the first tunnels were dug into the cliffs, which were expanded into a full-on mole empire during WW2.
Still, Dover is quite top-heavy when it comes to massive fortresses, seeing as there are a further two additional Napoleonic bastions the opposite side of town. The general idea was to have overlapping gun placements, so if any one position fell to invaders the other two could level it before it posed too much trouble. All three positions (Dover Castle and two on the Western Heights) are also linked by a warren of tunnels that run under the town. A lot of them are lost and collapsed, but if you know where the entrances are there's still a fair bit to see.