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.