A folly is in the eye of the beholder.

Follies, Grottoes and Garden Buildings, by Gwyn Headley and Wym Meulenkamp.

Folly

A name given to any costly structure considered to have shown folly in the builder,

- Shorter Oxford English Dictionary

But it's much, much more than that. The folly, particularly in Britain, is an attitude, a statement, a style, a fashion, a passion, a different world. . .

Follies may be found all over the world, but the British were first to recognise their worth and importance. . .

Gwyn Headley and Wim Meulenkamp have spent over thirty years reasearching 'rogue architecture'. They are co-founders of the Folly Fellowship.

Above text taken from the blurb on the back cover of Follies Grottoes and Garden Buildings by Headley and Meulenkamp, Aurem Press, 1999

"The Best European Folly of the 20th Century" - The Folly Fellowship

Of The Forbidden Corner - ' . . . a unique labyrinth of tunnels, chambers, follies and surprises created in a four acre garden in the heart of the Yorkshire Dales.'

http://www.yorkshirenet.co.uk/theforbiddencorner/