List of octagonal buildings and structures
Octagon buildings and structures are characterized by an octagonal plan form, whether a perfect geometric octagon or a regular eight-sided polygon with approximately equal sides. The oldest surviving octagon-shaped building is the Tower of the Winds in Athens, Greece, which was constructed circa 300 B.C.; the Pharos also had octagonal design elements but no longer stands.
Source: Wikipedia — List of octagonal buildings and structures (CC BY-SA 4.0)