Castel Del Monte
Concierge.com's insider take:
This eight-sided castle, which rises on a remote hilltop some miles inland from Trani and Barletta, is one of history's great puzzles. We know that it was commissioned by Holy Roman Emperor Frederick II and built between 1229 and 1249, but its function is a mystery. It was not built to defend anything, and there is no town or strategic crossroads nearby. Like Stonehenge and the Pyramids, the castle has attracted its share of theories: Some see it as a huge astronomical clock, others as a monument to the golden mean. The man who had it built was certainly capable of such a gesture: A ruler who bridged Christian, Greek, and Arab cultures, Frederick liked to unwind by writing poetry, philosophical tracts, and even a treatise on falconry.