The Hellenistic Period of Ancient History is generally though of as the time between the Classical Period (5th century BC) and the onset of the Roman period (1st century BC). It is characterized by Macedonian rule, brought about by the military exploits of Philip II, and later by his son, Alexander the Great. Both Philip (who ruled until 336 BC) and his son Alexander held the Greeks, particularly the Athenians, in high regard because of their affinity for art, culture, and intellectual pursuits. This respect would be beneficial to Athens during the tumultuous period of Macedonian conquer and rule.
In 338 BC, Philip of Macedon solidified his effort to control mainland Greece when he defeated Athens and Thebes at the Battle of Chaironeia at Boeotia. Because of Philip's respect for Athens, he treated them with relative fairness. However, the Athenians were understandably averse to the new way of things, and were likely frightened of losing their precious democratic political structure. Nonetheless, Athens was able to recoup the fortunes (lost at the hand of Sparta) somewhat during this time, thanks to Philip basically leaving them alone.
During this recovery period, much building occurred in Athens. Most of the building was not occurring in the Agora. A new Fountainhouse was constructed, as well as a water clock, or klepsydra. Both the water clock and the fountainhouse obtained water from an aqueduct that was built in the second half of the 4th century BC. The water clock is an interesting and important example of the innovations of Greek engineering, demonstrating their knowledge of flow, gravity, gears, and time calculation.
When Alexander died in 323 BC, most of Greece (including Athens) attempted to revolt against the Macedonians, and regain their independence. The conflict lasted less than a year, and resulted in Athens and Greece not regaining its sovereignty. Athens had forever lost the grandeur and importance it once held politically.
The 3rd century BC is marked by civil wars in the city. This likely drained what little resources (both finances and manpower) Athens had, and the Agora saw really no improvements or additions during this time. However, the Agora did hold its position as an important gathering place for the Athenian people.
The 2nd century BC saw the emergence of a new power, which came to the aid of Athens: Rome. Rome, having been victorious over the Macedonians at the beginning of the 2nd century (197 BC), provided Athens with the opportunity to recover yet again. And from this point, we will see more great things come out of Athens.