In order to investigate those spires I tried running 5 million iterations of training with a smaller network (10px per cell rather than 7px previously used). The result is quite interesting, but the neighbourhood size is too large, causing the sky to dominate significantly. There is still a tendency for clusters to grow in the upper left direction (causing spires) that I cannot explain. The light-coloured spires in the previous post originate at pixels with high degrees of brightness surrounded by darkness, but in the image below we can see they are also present in lower contrast areas (e.g. the sky).