Folly - 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.
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.
http://en.wikipedia.org/wiki/Folly
Key words being extravagant, useless, foolishness or madness, and most importantly partially in ruins.
Hence - "Opie's Folly"