Definition of "follies"

It has been a long time since new follies were springing up across the great estates of Britain. But the owners of Doddington Hall, in Lincolnshire, have brought the folly into the 21st century, by building a 30ft pyramid in the grounds of the Elizabethan manor.

2014 September 7, “Doddington's garden pyramid is a folly good show: The owners of a Lincolnshire stately home have brought the folly into the 21st century, by building a 30ft pyramid ”, in The Daily Telegraph‎, London