Folly
Visionary art structures frequently blur the line between artwork and folly, if only because it is rather often hard to tell what intent the artist had. The word "folly" carries the connotation that there is something frivolous about the builder's intent, and it is hard to say whether a structure like the Watts Towers was constructed "seriously". Some works (such as the massive complex by Ferdinand Cheval) are considered as follies because they are in the form of useful buildings, but are plainly constructions of extreme and intentional impracticality.