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 known octagon-shaped building is the Tower of the Winds in Athens, Greece, which was constructed circa 300 B.C. Octagon houses were popularized in the United States in the mid-19th century and there are too many to list here, see instead List of octagon houses. There are also octagonal houses built in other times and cultures.