The Sanctuary of Zeus Polieus was a no walled open-air sanctuary dedicated to Zeus Polieus (city protector)

Zeus

In the ancient Greek religion, Zeus was the "Father of Gods and men" who ruled the Olympians of Mount Olympus as a father ruled the family. He was the god of sky and thunder in Greek mythology. His Roman counterpart is Jupiter and his Etruscan counterpart is Tinia.Zeus was the child of Cronus...

 around 500 BC on the Acropolis of Athens

Acropolis of Athens

The Acropolis of Athens or Citadel of Athens is the best known acropolis in the world. Although there are many other acropoleis in Greece, the significance of the Acropolis of Athens is such that it is commonly known as The Acropolis without qualification...

, sited to the Erechtheion's east. None of its foundations have been discovered and its trapezoid plan and many entrances have been worked out from rock cuttings on the acropolis. The eastern area of the sanctuary is thought to have housed the oxen for the annual Bouphonia

Buphonia

In ancient Greece, the Buphonia denoted a sacrificial ceremony performed at Athens as part of the Dipolieia, a religious festival held on the 14th of the midsummer month Skirophorion— in June or July— at the Acropolis. In the Buphonia a working ox was sacrificed to Zeus Polieus, Zeus protector of...

or ox-sacrificing. Its main entrance had a pediment.

Source

• http://www.ancient-greece.org/architecture/zeus-polieus.html