Can you see something odd in the photo below?
folly1
Let’s get closer.
folly3
You might think, from this distance, that this building could actually be useful in some way. Then you walk there (thanks Alison for the arty shot!) :
folly
And you realise it’s an arch, on top of a hill. It is called a folly, which, according to the OED, is a popular name for any costly structure considered to have shown folly in the builder. This particular one is in Slindon, West Sussex, but there are a lot of them all around Britain. They were built for various reasons: to enhance the landscape, for contemplation, banqueting, and to provide work to the local population.
Catalogue of follies all over the UK