Copyright 2017 - The Friends of Mostert's Mill is a Non-Profit Organisation. NPO# 120-838
The commonest type of windmill in South Africa is the ‘tower-mill’, which has a cylindrical or truncated-cone of brick or stone surmounted by a thatched or wooden cap. The cylindrical tower-mill probably originated in France as early as the twelfth century, and could well have had a fixed cap (roof), which meant that the sails were likewise fixed and facing the prevailing wind.
 
In 1896 Alys Fane Trotter, an accurate draughtsman, made a pencil sketch of a cylindrical stone tower surmounted by a conical thatched cap at Broekman near the Salt River mouth:
 
 
The earliest illustration of a slightly tapered tower-mill is one of a Normandy mill depicted in an illuminated manuscript of between 1430 and 1440. The cylindrical tower was replaced by the truncated-cone, probably by Dutch millwrights in the sixteenth century.
 
The usual type of tower-mill at the Cape had sloping walls and a cap which could be rotated. The windshaft was horizontal and the sails turned vertically which necessitated the tower to be cylindrical to permit the rotation of the cap and the sweep of the sails.
 
By setting the windshaft at an angle compensation was afforded for the weight of the sails and this resulted in the tower being built in the familiar truncated-cone form and the sails could rotate clear of the walls.