Fortress plan according Euclid

Deep reasons link shape and geometry to the planning and the layout of Renaissance’s fortresses. In the 15th century, the diagram of many of them has a pentagonal shape. The pentagon is tied to the golden section and one can construct it starting from the golden triangle and its gnomon. To draw the approximated golden triangle, one establishes the relationship between the sides by means Fibonacci’s numbers, for example 5 and 8. The authors verify this relationship between numbers, shape and size in the layout of Castel Sant’Angelo (Rome).