Folly

Needle's Eye, Wentworth, South Yorkshire

Apollo Pavilion, Peterlee, County Durham

White Nancy, Bollington, Cheshire

Broadway Tower, Broadway, Worcestershire

Peterson's Folly, Sway, Hampshire

The Dunmore Pineapple, Airth, Scotland

folly (ˈfɒlɪ) 

— n  , pl –lies

1.the state or quality of being foolish; stupidity; rashness

2.a foolish action, mistake, idea, etc

3. a building in the form of a castle, temple, etc, built to satisfy a fancy or conceit, often of an eccentric kind

4.(plural) theatre an elaborately costumed revue

5. archaic

a. evil; wickedness

b. lewdness; wantonness

[from Old French folie madness, from fou mad; see fool]

Definition from Collins English Dictionary- Complete and Unabridged © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003