Caerphilly has the second biggest castle in Britain. It was built by the English to annoy the Welsh. The Welsh attacked it but couldn't take it. After the pacification of Wales it became virtually redundant. It saw some action in the early 14th century war between Edward II and Queen Isabella and then fell into disuse. By the time of the Civil War it was too ruinous to support a garrison.
Because it was only in use for the shortest time it was never seriously upgraded. It remains (if you discount the damage and some tinkering with details) a near-perfect example of 13th century military architecture.
It also has a tower- you can see it to the left of the keep (above)- which leans at a more extreme angle than the Tower of Pisa. No-one knows how this came about. It could have been something an attacking army did or simply the result of natural subsidence.
The BBC have been using the castle as a set for a programme called Young Dracula. They'd finished filming when we were there but parts of the building were still enhanced with fake gargoyles or draped with fake cobwebs. I fastidiously avoided getting any film props into my shots and now I regret it.