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 usual garden buildings.
Eighteenth-century English landscape gardening and French landscape gardening often featured mock Roman temples, symbolising classical virtues. Other 18th-century garden follies represented Chinese temples, Egyptian pyramids, ruined medieval castles or abbeys, or Tatar tents, to represent different continents or historical eras. Sometimes they represented rustic villages, mills, and cottages to symbolise rural virtues.  Many follies, particularly during times of famine, such as the Great Famine in Ireland, were built as a form of poor relief, to provide employment for peasants and unemployed artisans.
In English, the term began as "a popular name for any costly structure considered to have shown folly in the builder", the Oxford English Dictionary's definition.  Follies are often named after the individual who commissioned or designed the project. The connotations of silliness or madness in this definition is in accord with the general meaning of the French word foliecode: fra promoted to code: fr ; however, another older meaning of this word is "delight" or "favourite abode".  This sense included conventional, practical buildings that were thought unduly large or expensive, such as Beckford's Folly, an extremely expensive early Gothic Revival country house that collapsed under the weight of its tower in 1825, 12 years after completion.
As a general term, "folly" is usually applied to a small building that appears to have no practical purpose or the purpose of which appears less important than its striking and unusual design, but the term is ultimately subjective, so a precise definition is not possible.
The concept of the folly is subjective and it has been suggested that the definition of a folly "lies in the eyes of the beholder".  Typical characteristics include:
Follies began as decorative accents on the great estates of the late 16th and early 17th centuries, but they flourished especially in the two centuries which followed. Many estates had ruins of monastic houses and (in Italy) Roman villas; others, lacking such buildings, constructed their own sham versions of these romantic structures.
However, very few follies are completely without a practical purpose. Apart from their decorative aspect, many originally had a use which was lost later, such as hunting towers. Follies are misunderstood structures, according to The Folly Fellowship, a charity that exists to celebrate the history and splendour of these often neglected buildings.[ citation needed ]
Follies (French : fabriques) were an important feature of the English garden and French landscape garden in the 18th century, such as Stowe and Stourhead in England and Ermenonville and the gardens of Versailles in France. They were usually in the form of Roman temples, ruined Gothic abbeys, or Egyptian pyramids. Painshill Park in Surrey contained almost a full set, with a large Gothic tower and various other Gothic buildings, a Roman temple, a hermit's retreat with resident hermit, a Turkish tent, a shell-encrusted water grotto and other features. In France they sometimes took the form of romantic farmhouses, mills and cottages, as in Marie Antoinette's Hameau de la Reine at Versailles. Sometimes they were copied from landscape paintings by painters such as Claude Lorrain and Hubert Robert. Often, they had symbolic importance, illustrating the virtues of ancient Rome, or the virtues of country life. The temple of philosophy at Ermenonville, left unfinished,  symbolised that knowledge would never be complete, while the temple of modern virtues at Stowe was deliberately ruined, to show the decay of contemporary morals. 
Later in the 18th century, the follies became more exotic, representing other parts of the world, including Chinese pagodas, Japanese bridges, and Tatar tents. 
The Great Famine of Ireland of 1845–1849 led to the building of several follies in order to provide relief to the poor without issuing unconditional handouts. However, to hire the needy for work on useful projects would deprive existing workers of their jobs. Thus, construction projects termed "famine follies" came to be built. These included roads in the middle of nowhere, between two seemingly random points, screen and estate walls, piers in the middle of bogs, etc. 
Follies are found worldwide, but they are particularly abundant in Great Britain. 
Sir William Chambers was a Swedish-Scottish architect, based in London. Among his best-known works are Somerset House, and the pagoda at Kew. Chambers was a founder member of the Royal Academy.
Studley Royal Park including the ruins of Fountains Abbey is a designated World Heritage Site in North Yorkshire, England. The site, which has an area of 800 acres features an 18th-century landscaped garden, some of the largest Cistercian abbey ruins in Europe, ruins of a Jacobean mansion and a Victorian church designed by William Burges.
Sanderson Miller was an English pioneer of Gothic revival architecture and landscape designer. He is noted for adding follies or other Picturesque garden buildings and features to the grounds of an estate.
Prior Park Landscape Garden surrounding the Prior Park estate south of Bath, Somerset, England, was designed in the 18th century by the poet Alexander Pope and the landscape gardener Capability Brown, and is now owned by the National Trust. The garden was influential in defining the style known as the "English landscape garden" in continental Europe. The garden is Grade I listed in the Register of Historic Parks and Gardens of special historic interest in England.
The English landscape garden, also called English landscape park or simply the English garden, is a style of "landscape" garden which emerged in England in the early 18th century, and spread across Europe, replacing the more formal, symmetrical French formal garden which had emerged in the 17th century as the principal gardening style of Europe. The English garden presented an idealized view of nature. Created and pioneered by William Kent and others, the “informal” garden style originated as a revolt against the architectural garden and drew inspiration from paintings of landscapes by Salvator Rosa, Claude Lorrain, and Nicolas Poussin.
The Dessau-Wörlitz Garden Realm, is a cultural landscape and World Heritage Site in Germany, located between the city of Dessau and the town of Wörlitz in Central Germany. One of the first and largest English parks in Germany and continental Europe, it was created in the late 18th century under the regency of Duke Leopold III of Anhalt-Dessau. Today, the cultural landscape of Dessau-Wörlitz encompasses an area of 142 km2 (55 sq mi) within the Middle Elbe Biosphere Reserve in the German state of Saxony-Anhalt. Because of its exceptional landscape design and testimony to the ideals of the Age of Enlightenment, the Dessau-Wörlitz Garden Realm was designated as a world heritage site in 2000.
Pontypool Park is a 150-acre (0.61 km2) park in Pontypool, Torfaen, Wales. The park was formerly the grounds of Pontypool House and was laid out in the closing years of the 17th century for John Hanbury, an ironmaster, who is closely associated with Japanware. The grounds were purchased by the local authority in 1920, while the estate house was leased, and later sold, to the Sisters of the Holy Ghost to become St. Alban's RC High School. The former stables now house the Torfaen Museum. The grounds contain a number of structures including a double ice house, the Folly Tower and the Shell Grotto. The park is entered through the Pontymoile Gates. The gates, the grotto and the stables are all Grade II* listed structures, while the former hall and the ice house are listed Grade II. The park itself is designated at Grade II* on the Cadw/ICOMOS Register of Parks and Gardens of Special Historic Interest in Wales.
Port Quin grid reference SW971805 is a small cove and hamlet between Port Isaac and Polzeath in north Cornwall, England.
The Folly Fellowship is a UK charity and company limited by guarantee. It was created in 1988 by Gwyn Headley, Wim Meulenkamp and Andrew Plumridge as an amenity society to protect, preserve and promote awareness of Britain's follies, grottoes and garden buildings. It organises trips throughout the year to follies and holds an annual garden party at a follied garden where the highlight is the cutting of a cake formed in the shape of one of the follies in the garden. Members also receive a range of publications, including three Magazines, each giving information about follies in different depths. Folly Fellowship members include architects, people who live in follies, people who build follies and other interested persons.
Clytha Castle is a folly near Clytha between Llanarth and Raglan in Monmouthshire, south east Wales. Dating from 1790, the castle was built by William Jones, owner of the Clytha Park estate as a memorial to his wife, Elizabeth, who died in 1787. The castle is an example of the Gothic Revival and comprises three towers, of which two are habitable, and linking, castellated curtain walls. Long attributed to John Nash, recent research has confirmed that the architect was John Davenport of Shrewsbury. The folly has views towards the Sugar Loaf and Skirrid mountains on the easternmost edge of the Brecon Beacons National Park. Described by the architectural historian John Newman as one of the two "outstanding examples of late eighteenth century fanciful Gothic in the county", Clytha Castle is a Grade I listed building.
The Kymin,, is a hill overlooking Monmouth, in Monmouthshire, Wales. It is located approximately one mile east of Monmouth, on the eastern side of the River Wye and adjacent to the border with the Forest of Dean and England. The summit of the hill, about 800 feet above sea level, is known for its neo-classical monuments, the Roundhouse and the Naval Temple, built between 1794 and 1800. It is registered on the Cadw/ICOMOS Register of Parks and Gardens of Special Historic Interest in Wales. The site is within a designated Area of Outstanding Natural Beauty (AONB) and is owned by the National Trust.
The French landscape garden is a style of garden inspired by idealized romantic landscapes and the paintings of Hubert Robert, Claude Lorrain and Nicolas Poussin, European ideas about Chinese gardens, and the philosophy of Jean-Jacques Rousseau. The style originated in England as the English landscape garden in the early 18th century, and spread to France where, in the second half of the 18th century and early 19th century, it gradually replaced the rigidly clipped and geometrical French formal garden.
Sham Castle is a folly on Claverton Down overlooking the city of Bath, Somerset, England. It is a Grade II* listed building. It is a screen wall with a central pointed arch flanked by two 3-storey circular turrets, which extend sideways to a 2-storey square tower at each end of the wall.
John Norris was an English merchant and a member of the landed gentry. He was High Sheriff of Buckinghamshire in 1775.
Gwyn Headley is a British historian and writer.
A shell grotto is a type of folly, a grotto decorated with sea shells. The shell grotto was a popular feature of many British country houses in the 17th and 18th centuries. It suited the Baroque and Rococo styles and often represented the mimicry of architectural features from the Italian Renaissance. The idea of a grotto was originally a means to enhance a dank undercroft, or provide an antechamber before a piano nobile, but later it became a garden feature independent of the house, sometimes on the edge of a lake, with water flowing through it.
A folly tower is a tower that has been built as an architectural folly, that is, constructed for ornamental rather than practical reasons. Folly towers are common in Britain and Ireland, and often do have some practical value as landmarks, or as viewpoints, unlike other types of folly.
The Pepperbox, also known as Eyre's Folly, is a folly tower that stands near the highest point on Pepperbox Hill, the peak of a chalk ridge about 5 miles (8 km) southeast of the city of Salisbury, Wiltshire, England. Built in 1606 by Giles Eyre, the folly is a three-storey hexagonal tower constructed of brick, with its entrances and windows blocked up. The building's original purpose is unknown, though theories include that it was built to provide Eyre with views of Longford Castle or to provide local landowners' wives, including Eyre's wife Jane, with a lookout tower to watch the hunt. The tower is considered one of the oldest follies, and is a Grade II listed building. The tower and hillside are owned by the National Trust.
Shadwell Court, Brettenham, Norfolk, England is a country house dating originally from the 18th century. Built for the Buxton baronets, the house was massively enlarged in two stages in the 19th century; in 1840-1842 by Edward Blore and then in 1856-1860 by Samuel Sanders Teulon. The house and grounds now form part of the Shadwell Nunnery Stud, owned by Hamdan bin Rashid Al Maktoum until his death in March 2021. Shadwell Court is a Grade I listed building. In 2019 the court was included in the Heritage at Risk Register due to concerns over the deterioration of its fabric.