The ruins of Tholos Temple are all that remains of the Sanctuary of Athena Pronaia.  The Sanctuary was the gateway to Delphi and may actually predate Delphi as a sacred place.  It was originally dedicated to the worship of an Earth Goddess, but was eventually occupied by Olympian deities, Athena in particular.  Tholos Temple, built in the early 4th century BC, has an unusual circular shape.