The temple of Zeus.
The tyrant Peisistratus began building the Temple of Olympian Zeus south of the Acropolis in 515 B.C. When he was overthrown the project stopped, and was completed only eight hundred years later by the Roman Emperor Hadrian. That took even longer than the National Cathedral in Washington D.C. which as a mere eighty years or so.
When completed there were 196 of these mammoth columns, of which only a dozen remain. It must have been gigantic.
It has a clear line of sight to the Acropolis, which is no doubt maintained by some kind of legal restriction. Good. It was visible from the Acropolis and the Acropolis is visible from it. It lies on flat ground near Syntagma Square.
If you examine the above picture carefully, Hadrian's Gate is in the foreground. It does not stand out very much in the picture but it is there.
The photograph below from the site brochure may be easier to see.
For more see http://odysseus.culture.gr/h/3/eh351.jsp?obj_id=2488