"A costly structure demonstrating the foolhardiness of its builder." is how the Encyclopedia of the Exquisite describes a folly and is largely true, but I think of it as more romantic than that - which is why I had Hamish Buchanan from "Highland Heat" have one on his property in The Highlands. They were often built as little palaces to bring a smile to a lovers face, even though they weren't all that habitable.
My cutie and I frequently visit Fountains Abbey and Studley Royal Water Garden in Ripon, Yorkshire, England and it has a few follies dotted around the extensive site. The one in the picture below is the best preserved and easiest to peak through the windows. In case you're wondering, it would make a lovely reading nook.
One of the ultimate folly dabblers is King Ludwig of Bavaria with his Neuschwanstein Castle, which literally translates to "new-swan-stone". It was built to resemble a 13th century palace like those found in a German folktale about a German prince who morphed into a swan, but Ludwig went a bit wild on the details:
By the way, Neushchwanstein, in case you don't already know, is the model for Sleeping Beauty's castle at Disneyland.
"It took fourteen carpenters more than four years to complete the carving in his bedroom alone. A trapdoor in teh dining room allowed the table to be lowered into the kitchen and set while the servants remained out of sight. The throne room was decorated in fantasy Bysantine style. A stalactite grotto off of Ludwig's study came complete with a waterfall and an electric "moon" moving through its phases."