Hmm. A village called San Miguel? I'd expect more ruins. A mill? I'm ruling nothing out. I wish (just a bit) that I'd walked the 'real' camino from Hontanas to Castrojeriz, rather than taking the easy route along the smooth, level, tarmac road. [Don't condemn me! I walked the real, i.e. signposted, camino in 2003, but I was using film in those days, and probably felt I could not spare a frame for that structure.] I could have had a closer look at the tower. Is it a tower, even? It seems too small in width (both ways) to have a staircase within it. It cannot be a water mill, as there is no stream nearby. Windmills are not common round there – at least, I've never seen one.