Horse Turning Mill
The horse is driving a horizontal wooden wheel which gears into a vertical one slung with buckets. The bucket chain passes down into a well.
This is one of several types of noria (or water lifting wheel) which used to be common in south-eastern Spain.
The railings and the glass barrier are, of course, modern embellishments designed to prevent sightseers from coming to harm. Fifty years ago, when the noria was still worked, the local children used to catch a ride on the clay buckets descending down into the well, enjoy a good dunking, and then ride up again with the laden buckets.