Athens after Piraeus, on a land-locked bay with a rich plain, was a strong prehistoric settlement but merged with Athens sometime before the 7th cent. BC. There was an important theatre of Dionysus there, and the sanctuary of Demeter and Kore was the site of many festivals of local or national importance (Eleusinia, Thesmophoria, Proerosia. Haloa, Kalamaia), but the fame of Eleusis was due primarily to the annual festival of the Mysteries, which attracted initiates from the entire Greek-speaking world. Within the sanctuary of the Two Goddesses the earliest building that may be identified as a temple is 8th cent. Its replacement by increasingly larger buildings (two in the Archaic period, two attempted but not completed in the 5th cent.), culminating in the square hall with rock-cut stands built under Pericles, the largest public building of its time in Greece, bears eloquent witness to the ever increasing popularity of the cult. The unusual shape of this temple reflected its function as hall of initiation (usually called Anaktoron, sometimes Telesterion). Destroyed by the Costobocs in AD 170, it was soon rebuilt under Marcus Aurelius, who also brought to completion the splendid entrance, a copy of the Propylaea on the Athenian Acropolis. In this he followed the imitative of Hadrian, who was primarily responsible for the physical renewal of the sanctuary in the 2nd cent. The sanctuary evidently ceased to exist after AD 395.
From: The Oxford Dictionary of Classical Myth and Religion, edited by Simon Price and Emily Kearns, Oxford University Press, Oxford, 2003, pp. 186-7.