How were sand, gravel and cement (the ingredients of concrete) carried up to those heights, and mixed and placed, and floated and trowelled?? And the steel bars hauled up and fitted and installed?
Unimaginable!!
If it were Russia, the answer would be political prisoners, most of whom were not expected to survive. And Walter Duranty would have been there to celebrate the triumph of inspired socialist labor.
But in Spain, I don't know.