Octagon Earthworks
also see Ohio Historical Society Octagon Earthworks webpage

           Octagon Earthworks are one of the most fascinating components of the Newark Earthworks.  It consists of a circular enclosure connected to an octagon by a short section of parallel walls.  The circular enclosure forms a nearly perfect circle 1,054 feet in diameter.  It only deviates from a perfect circle of that diameter by less than four feet.  It encloses an area of about 20 acres.  The most interesting feature of the circle is the so-called "Observatory Mound" located along the southwestern rim opposite the opening to the octagonal earthwork.  The Observatory is an elongated platform mound 170 feet long          Image Credit CERHAS Please click on the image for a closer view
and about 12 feet in height.  It appears to have been built across another opening into the circle consisting of a short segment of parallel walls.            

            The walls of the octagonal enclosure were each about 550 feet long and from five to six feet in height.  There were gateways or openings at each corner of the octagon varying from about 50 to 90 feet in width.  Each opening of the octagon is partially blocked by a rectangular or oblong platform mound about 100 feet long by 80 feet wide at the base and between five and six feet high.
The octagon itself encloses nearly 50 acres.
at right, the Observation Mound
 

Also see
THE NEWARK EARTHWORKS: A WONDER OF THE ANCIENT WORLD

WHAT ARE THE NEWARK EARTHWORKS