Disused mill at start of footpath to Castle O' Burrian

Disused mill at start of footpath to Castle O' Burrian is a castle, fort or tower in Orkney Islands, Scotland. Perhaps the mill stones still turn when nobody watches, grinding shadow instead of grain. Maybe the footpath marks the route of people who fed a tower that no longer stands. Imagine dropping a marble into the mill hole and hearing it return clangling back, angry at being noticed.