This was an old military Radar installation.   I think radar balls used to be on top of the round towers.

Nike or Hawk site?   Links:

 info on Nike Missiles Detailed Locations More about Nike  Some mention of Nike in Florida Nike Q&A Details of site near Key West

Aerial Photography  In color

Sites in Miami (http://ed-thelen.org/old_sites.html).  I wish I could read more of it.  But I think we have HM-95 and HM-40 found and documented.   I don't think this is to scale.