*I'm English and normally regard metric as the work of the Devil, although I concede it does have its uses in fine-scale modelling. And so I've used it here. Each strip is 7mm wide, with cuts made on one long edge every 4mm to a depth of 5mm to represent the individual tiles. The longest strip will run around the lower edge of the base area, with overlapping strips of decreasing length upwards from there, set about 5mm up from the edge of each lower run. Once these are fixed in place with contact adhesive, the whole will be painted (probably slate gray), given a black wash to pick out the "tiles" then varnished. A strip of tinfoil will be glued to the area between the tiling and the turret wall and painted to represent lead-sheet waterproofing.
*And then it's on to the door, the wall and gatehouse - and the actual paintwork for the tower.