The windmill at Aythorpe Roding is a type known as a post mill. The earliest kind was a fixed structure usable only when the wind was blowing in the right direction. Originally it had no roundhouse beneath it, to protect the trestles upon which the mill was supported from decay, but later types had an exterior housing which helped to preserve the trestles and provided storage space within.
In the Middle Ages the body of the mill, which contained the machinery which drove grinding stones and sails, was balanced on a central post and could be turned into the wind by means of a pole at the back of the mill. The oldest surviving example of this kind is at Bourn, Cambridgeshire, and was erected in 1636. In 1745 a method was patented which used a fantail and gears for keeping the mill facing into the wind. Many post mills were built upon a raised mound to gain extra wind-power.
Other types of mill are smock mills in which only the cap with the sails revolved and tower mills. The difference between the two was that the former was constructed of wood whereas a tower mill was constructed of either brick or stone.
[source: The Local Historians Encyclopedia]