The string courses were a measure to prevent rats climbing the walls and getting in through the pigeon access holes at the top.
The design is best described as similar to a wedding cake, rising in concentric layers, each smaller than the last. What makes the Rendall Doocot so interesting is that most doocots on Orkney were built as a rectangle, or to a square plan, with a lean-to roof.
Access is easy; just take the signed drive to Hall of Rendall off Gorseness Road, between Gorseness and the Tingwall ferry. There is a small parking area and the doocot is easily visible from the drive. Access is limited to exterior views, as the doocot is still used by local birds.
Please be careful not to alarm the birds when approaching the building. I thought the doocot was quite striking because of its unusual design, though it is a shame there was no internal access to see the nesting boxes.