Folly is also a synonym for foolishness; see stupidity. A folly is a silly error.
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.