This monumental gateway used to serve as the entrance to the sacred site Acropolis. It used to be guarded so that people who are not ritually clean could not enter the sanctuary. The structure is made of white Pentelic and gray Eleusinian marble.
Wikipedia https://en.wikipedia.org/wiki/Propylaea_(Acropolis_of_Athens)
Address (Unnamed Road), Αθήνα 10556, Greece
Coordinates 37°58'18.296" N 23°43'30.456" E
