In architecture, a folly is a building usually constructed strictly for aesthetic pleasure. Originally, buildings were made to provide shelter or to house people. Follies are just decoration; they no longer have a building's original function.They were first constructed to put accents into parks and estates. "Folly" is used in the sense of fun or light-heartedness, not in the sense of something ill-advised.

Broadway Tower, Worcestershire, England

Wimpole's Folly, Cambridgeshire, England, built in the 1700s to resemble Gothic-era ruins

Decorative ruin in Schönbrunn, the royal palace in Vienna

Decorative aqueduct in Arkadia, Poland