Octagonal architectures between religious and power symbols

In the past, the most classic geometries, for defines the layout of the architectural space was the circle, rectangular and square. However, there are octagonal layout architectures, which due to the very strong intrinsic meanings that this form possesses was used for particular architectural structures that had a strong symbolic value, of secular and religious power, often opposed to each other precisely through the particular shape of their octagonal layout. The octagonal geometric shape is full of esoteric meanings. Used to define the architectural space of numerous baptisteries, towers, castles and funeral monuments that are inextricably linked to number 8. For various peoples, the octagon has been a sacred geometric figure throughout history, so much so that it has been used for various architectures with an octagonal layout. Frederick II of Swabia, Leonardo da Vinci and many others drew inspiration from this particular shape, used extensively in Islamic architecture. The goal of this paper is to understand the meaning hidden behind this geometric figure when it is used for the definition of the architectural space, to catalog in chronological order the numerous architectures with this particular layout and to understand their placement in the historical, landscape, political and religious period, focusing on the Dome of the Rock, in Jerusalem, which best represents the meaning of octagonal architecture for the three major monotheistic religions.