Old Erechtheion Precinct
Around 465 BC, the Pre-Erechtheion Precinct was remodeled. One theory argues that this was to receive the Statue of Athena Polias. However, others advocate that the Opisthodomos of the Old Temple of Athena was restored precisely for the same purpose...
The construction of the Erechteion Temple, probably began before 430 BC and was completed in 406/405 BC. In 421 BC, even though construction had probably begun, older buildings could still be found on the precinct: the shrine of Athena Polias, the tomb of Kekrops, the altars of Poseidon-Erechtheus, Zeus Hypatos, Boutes and Hephaistos, the sacred olive tree, the sacred salt spring, the marks of Poseidon's trident... The Athenians called the Erechteion the "temple on the Akropolis in which the ancient statue is".
After the major destructions incurred during the Persian occupation of 480 BC, The Athenians seem to have built a complex of stoas and shrines in the precinct that would later host the Erechtheion.
Altar of Poseidon-Erechtheus
Originally, the altar of Poseidon-Erechtheus was probably located near the grave of Erechtheus which in turn must have been within an adyton since it was a mystery cult.
This archaic tomb was later included into the Erechtheion. Indeed, the Karyatid porch of the Erechtheion partly sits atop this tomb. Originally, in the Mycenaean period, the tomb was probably a simple mound of earth. Around 450 BC, a tall stele on a crepidoma possibly indicated its location.
Sacred Salt Spring
The spring was probably within an adyton since it was a mystery cult.
Shrine of Athena Polias
This shrine, not to be confused with the temple of Athena Polias, was included into the Erechtheion when this latter temple was built.
Brouskari, Maria; The Monuments of the Acropolis, Athens: Archaeological Receipts Fund, 2001
Camp, John M.; The Archaeology of Athens, Yale University Press, 2001, pp. 93-100
Hill, Ida Thallon; The Ancient City of Athens – Its Topography and Monuments, London: Methuen & Co. Ltd., 1953
Hurwit, Jeffrey M.; The Acropolis in the Age of Pericles, Cambridge: Cambridge University Press, 2004
Travlos, John, Pictorial dictionary of Ancient Athens, Books that matter, New York, 1971, pp. 213-214