The flat top is the same diameter as Stonehenge, which once stood on there.
It looked spiffing in those days. It was used as a pigpen as the original builders, the Baconite people, worshipped them.
The stones were subsequently removed and re-erected on Salisbury Plain, near to the Baconite feasting centre at Woodhenge. They found it easy as they had anti-gravity. If you look carefully you can see slight gaps between the lintels at Stonehenge, this being due to the fact the reconstructed version's diameter is 8 inches too big, the result of a failure of communication regarding the true length of a megalithic yard. The project manager paid the ultimate price and was buried at Amesbury in a cramped position, his grave being deliberately constructed 8 inches too short, a deliberate and brilliant ironic message to the future by the chief Baconmeister.
Henge in early English means pigpen. Hence, it's original name, Stane-pigpen. Archaeologists have supressed the Truth.