According to Webster's Dictionary a folly is a "foolish and useless but expensive undertaking". So what constitutes an architectural folly? In his book Follies, Shire Album 93, Jeffery W. Whitelaw notes that although they may take on different forms and styles follies are primarily constructed to be looked at and enjoyed. They may or may not have any purpose beyond being "aesthetic completions of the landscape". 

By far the most prolific folly builders in England were the wealthy landowners of the Eighteenth Century.  They had the land, labour force and budget for materials.  Many were influence by the classical architecture they saw on their grand tours of Europe.   Some may have wanted to leave their stamp on the landscape or extend the style of their country mansions into their parkland.  Whatever the motivation of their builders, follies are lasting reminders of the era when Georgian squires wielded power and influence.

The Folly Fellowship was founded in 1988 as a pressure group to protect, preserve, and promote follies, grottoes & garden buildings. Initially a group of enthusiasts keen to record what was at first seen as a peculiarly British aspect of architecture, it has grown into a serious conservation and consultative architectural heritage charity, while not losing sight of the basic idea that these buildings are fun - they were built for pleasure before purpose. 
Folly building is not unique to the United Kingdom. There are many interesting examples in Europe and beyond. The Désert de Retznear Paris, France, is the best surviving example of the eighteenth-century Jardin Anglo-chinois, or folly garden, and was designed and constructed between 1774 and 1789 by a wealthy aristocrat, François Racine de Monville. 

The similarities between Monville and Fuller are intriguing. Both were considered eccentrics who were well connected to many notable figures in their day. Both constructed a pyramid, an obelisk and a classical temple. 

Brightling Needle
Sugar Loaf
Désert de Retz
John 'Mad Jack' Fuller
Fuller's Follies
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