To the S of the Acropolis, in the area between the Hill of the Muses and the Ilissos river, numerous prehistoric remains have been found. These finds confirm not only the location, but also the extent of the most ancient city, just as Thucydides (2.15.3-6) delineated it, on the S side of the Acropolis. It is precisely in this area that the very ancient shrines are to be found: the Olympieion, the Pythion, and the Shrine of Dionysos in the Marshes, along with the Kallirrhoe spring and the Enneakrounos fountain.
According to Pausanias (1.18.8) the first temple to Olympian Zeus was erected by Deukalion. Over this Peisistratos the Younger laid the foundations of a large poros Doric temple but never finished it. This temple was to have had not only the same dimensions but also the same general appearance as the Hellenistic-Roman temple. In 174 B.C. Antiochos Epiphanes started the construction of a marble Corinthian temple which was finished in A.D. 131-132 under Hadrian. At the same time a great peribolos wall was built around the temple and in its NW corner is still preserved the gate in honor of Hadrian which set the boundary between the old city and the new one founded by Hadrian.
Within the Themistoklean wall and to the S of the Olympieion the following buildings have been discovered: the poros Temple of Apollo Delphinios (450 B.C.) which, according to tradition, was built on the site of a very ancient temple, the court of the Delphinion which is dated to 500 B.C., the Temple of Kronos and Rhea from the period of the Antonines, and the Panhellenion (A.D. 131/2). Next to the wall of the city, but outside it, should be the site of the Pythion, according to a number of relevant inscriptions which have been discovered. A small stoa SW of the Olympieion dating to the mid 6th c. B.C. must be identified as the court of the Palladion. To the S of it the discovery of an ancient boundary stone in situ confirms the site of the Shrine of Kodros, Neleus, and Basile, and associated with this and in front of it (according to the inscription IG I2 94), the Sanctuary of Dionysos in the Marshes.
On the other bank of the Ilissos, near the Church of St. Photini, is the site of Kynosarges, where the ruins of the Gymnasium, built in A.D. 134 by Hadrian, were found. The little mid 5th c. B.C. Ionic temple of the Ilissos now vanished should be attributed to Artemis Agrotera, and the ruins which have been discovered next to the Ilisos, to the Metroon in the Fields. Somewhat to the N, in the hollow between the hills by the Ilissos river, the first stadium was built by Lykourgos. On the same site Herodes Atticus built the new Stadium in A.D. 143-44. This was restored in 1896 for the holding of the first Olympic Games. North of this was the site of the Shrine of Herakles Pankrates, and between the Ilissos and the E side of the city was the Gymnasium of the Lykeion and the Gardens of Theophrastos.
Info: Princeton Encyclopedia
(Princeton Encyclopedia of Classical Sites, from Perseus Project)