In architecture, a folly is an extravagant, useless, or fanciful building, or a building that appears to be something other than what it is.
The term comes from the fact that such structures have often been dubbed "[name of architect or builder]'s Folly", in the sense of foolishness or madness.
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Follies are usually found in parks or large grounds of houses and stately homes; they may sometimes have been deliberately built to look partially in ruins. They were especially popular from the end of the 16th century to the 18th century.