in that the aperture segment 10 ends up being of the same thickness as the neighbouring tower rings 22 above and beneath the sharpening 11 on the top and bottom edges could advantageously be made before the aperture segment 10 is rolled into the circular shape in another embodiment of the invention the sharpening 11 could be non linear e g having the shape of a part of an ellipse and or the edge between the sharpening 11 and full material thickness of the aperture segment 10 could be rounded fig4 illustrates the tower part 6 comprising a wall segment 9 and an aperture segment 10 with a door aperture 7 and a door 8 in a horizontal cross section through the middle of the door aperture 7 in an embodiment of the invention all four edges on both the concave and the convex side of the aperture segment 10 are provided with a sharpening 11 in an embodiment of the invention the sharpening 11 on the side edges of the aperture segment 10 results in the aperture segment 10 ends up being of the same thickness as wall segment 6 because of the way a traditional rolling machine known in the art functions the sharpening 11 on the side edges.