You may have noticed how a lot of the older, small airfield type towers (generally quite low in height) have vertical windows - these do function quite well and are cheaper to build. However, they can be subject to a lot of reflections at night - but during the daytime are fine. New towers do however tend to be built with sloping windows and also tend to be taller - sloping windows certainly make for better aerodrome visibility in this case. The more interesting thing is the variety of cleaning devices that are used to clean the exterior of tower windows....