I have been fascinated by Dover Castle for as long as I can remember.
It has a Norman Keep that is over nine hundred years’ old, medieval underground defences, Napoleonic defences, WW2 fortifications and a Cold War Nuclear regional government installation from the 60’s .

The picture is of part of the Napoleonic underground workings that extend outside the castle walls.
On the end of the tunnels is a Guardroom, should this guardroom be overrun, the guards would fall back to this position, where they can fire to either side along the moat.
From the platform at the end, they can spy through a hole on the passage leading in from the guardroom. This passes an area light by a circle of daylight and then drops down a spiral staircase and into a tunnel under the floor we can see. The raised door at the end can be dropped vertically so as to block off this tunnel. When would be invaders find the tunnel blocked they return to the staircase where boiling oil is poured on them through the larger right hand shoot.

When I have explained all this to my family in the passed I have sometimes gathered quite a crowd of interested people.