Castel del Monte – Medieval Architectural Wonder A medieval castle on the southeastern coast of Italy is famous for its symmetrical repetition of octagonal forms in the citadel’s layout, the towers, and the courtyard. A recent study reveals the design provenance, which details the inspiration, the purpose, the location choice, and the innovative science and technology employed for this one-of-a-kind architectural marvel. From conception to construction, it is a story of fascinating discoveries highlighting the designers’ new vision and the builders’ achievements and shortcomings. The castle represents the novel solution for securing Gothic vaults, albeit within an octagonal plant, which gives rise to the constellation of octagons in a fractal pattern. More significantly and concealed in the design is a new approach to defining the structures; it is based on deductive reasoning and mathematically defined dimensions, a glimpse of modern science. Surprisingly, the plant design is a geometric algorithm with a mathematical facet, much like an origami procedure. The builders laid the foundation from the designer’s plant layout with amazing precision, which is why we can decipher the medieval plant design. Inserting the plant dimensions in the plant design algorithm reveals the medieval unit of measurement at the castle, a slight variant of the Roman foot. The stonework also tells us that the construction was beset by the same complications often found in modern projects: plan changes, cost overruns, and planning shortcomings. An amazing finding is the construction blunder of short columns on the upper floor that was resolved by adding another pedestal camouflaged as bench seats extended around the room. Castel del Monte is more than a singular octagonal citadel; it is a testimonial of innovative design and construction concepts, a flash of enlightenment in the middle of the “Dark Ages.”