In architecture, a folly is a building constructed primarily for decoration, but suggesting through its appearance some other purpose, or of such extravagant appearance that it transcends the range of garden ornaments usually associated with the class of buildings to which it belongs.
18th century English gardens and French landscape gardening often featured mock Roman temples, symbolizing classical virtues. Other 18th century garden follies represented Chinese temples, Egyptian pyramids, ruined abbeys, or Tatar tents, to represent different continents or historical eras.
info: Wikipedia
Broadway Tower, Worcestershire, England
By Saffron Blaze – Own work, CC BY-SA 3.0, Link
The Dunmore Pineapple in Scotland
By giannandrea – Own work, CC0, Link
Modern reconstruction of the Turkish Tent, a permanent structure at Painshill, Surrey.
4 WyrdLight.com, CC BY-SA 2.5, Link
Rushton Triangular Lodge, Northamptonshire, England, built in the late 16th century to symbolize the Holy Trinity.
By Photographer: SiGarb – Low-res version of my own digital photograph, Public Domain, Link
Wimpole’s Folly, Cambridgeshire, England, built in the 1700s to resemble Gothic-era ruins.
By Users Tysto, Nickarse2412 on en.wikipedia – Own work, CC BY-SA 3.0, Link
The Beacon: One of the remaining follies at Staunton Country Park originally commissioned by George Thomas Staunton and designed by Lewis Vulliamy.
By Geni – Photo by user:geni, GFDL, Link
Casino at Marino, Dublin, Ireland
By Anonymous – Own work, Public Domain, Link
Hagley Castle is in the grounds of Hagley Hall. It was built by Sanderson Miller for George, Lord Lyttelton in the middle of the 18th century to look like a small ruined medieval castle.
By Paul Brooker, CC BY-SA 2.0, Link
Lija Belvedere Tower in Malta
By Frank Vincentz – Own work, CC BY-SA 3.0, Link
e MORFES 
I suppose in that particular period in history the construction of a folly was a sign to the world that you had more money than you knew what to do with, rather like the supercars of today. Thank you for the article – there are not many follies at my end of England, though the West Country of my youth was littered with them.
An interesting material, I really liked the Turkish tent in Surrey and the Holy Trinity of the 16th century.