In everyday language, the word “folly” usually refers to a foolish action or object. But to an architectural historian, it has a more precise meaning: a folly in that sense is any decorative building that has little or no practical use.

True architectural follies are relatively rare in North America. (Building follies in the more general sense, of course—as exemplified by oversized, haphazardly insulated, and poorly air-sealed residential structures—are not uncommon at all.) But in much of Europe—and especially Great Britain and Ireland—there’s a long tradition of architectural...

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