Rotunda at Stainborough Park near Barnsley in Yorkshire
Wikimedia Commons
A folly, sometimes called a focal point or eyecatcher, is an ornamental structure with no practical purpose that was built to enhance a designed garden or parkland landscape.
Follies were particularly fashionable during the 18th and early 19th centuries, when they were incorporated into the landscape of English country house gardens by wealthy landowners. They took many forms: scaled-down versions of temples, pyramids, mock castles, gothic ruins, arches, towers and obelisks.[1]