From Project Athinai
Demeter and Kore had their main sanctuary at Eleusis. However, after the integration of Eleusis into the Athenian State, a smaller sanctuary, called the Eleusinion, was built in Athens around 480 BC. It was located on the east side of the Panathenaic Way to the south of the Agora, on the north slope of the Akropolis. An earlier walled open-air sanctuary already existed on the same location since around 550 BC.
Each year, immediately after the celebration of the Mysteries at Eleusis, the Boule assembled within the walled precinct of the Eleusinion.
The precinct consisted of a level terrace surrounded by walls. The 0.90 m thick precinct wall, dating from around 550 BC, was made of limestone in polygonal style. The north and south walls were 6 m high. The entrance to the sanctuary was located at the southwest corner. The precinct measuring some 40 m by 20 m was delimited to the north and south by streets connected to the Panathenaic Way, and to the west by the Panathenaic Way.
The temple occupied the middle of the sanctuary and probably faced south. It consisted of a rectangular cella with a smaller treasure room behind it to the north and measured 11.00 m by 17.70 m. The building rested on a foundation of Kara limestone. The shrine was probably filled with ritual vases dedicated to the goddesses. Standing on a high terrace, the temple was visible from the road coming from Eleusis.
There is still a doubt that this building might be the temple to Triptolemos rather than the Eleusinion proper. According to this theory the remains of the Eleusinion would still be unexcavated.
A 2 m by 15 m stelai was erected in the precinct to remind of the sanctions against Alkibiades and his friends who had mutilated the Herms and parodied the Mysteries. However, the mutilation of the Herms occured only in 415 BC.
Around 325 BC, a marble propylon was erected at the southwest corner in place of the former entrance.
American School of Classical Studies at Athens; The Athenian Agora: A Guide to the Excavation and Museum, 1990
Travlos, John, Pictorial dictionary of Ancient Athens, Books that matter, New York, 1971, p. 198