5Castel del Monte, Italy
A true architectural marvel, Castel del Monte is a unique 13th-century fortress located in southern Italy. Commissioned by Emperor Frederick II, this octagonal fortress stands out for its geometric precision and design. Unlike traditional medieval fortresses, Castel del Monte has no clear defensive purpose, and its exact function remains a mystery. Its perfect octagonal shape, with eight towers at each corner, reflects a blend of Islamic, Romanesque, and Gothic architectural styles. The fortress’s striking symmetry and mysterious origins have earned it a place as a UNESCO World Heritage Site, and it remains one of the most intriguing and impressive fortresses in Europe.