The exterior of Athena's shield

In the middle, the head of the Gorgon Medusa (also found, a bit redundantly, in the middle of the aegis, Athena's breastplate). The sight of Medusa's head was said to turn one to stone, so she was a favorite on shields.

The battle around Medusa is an attack on the Athenian acropolis by the Amazons. It seems that after the Persian sack of Athens in 480, the Athenians often made Amazons mythological stand ins for the Persians. One sign of this is the fact that prior to around 480 battles between Greeks and Amazons were usually set in the home of the Amazons. The Athenians liked to consider themselves manly, self-controlled, free warriors, while their Persian opponents were effeminate, emotional, slaves. For the most part, Greek art did not represent historical events directly; by making use of mythological analogues, the Greeks sought to impose a universal message on their historical victories. Free men defeat effete slaves every time.

The artwork on the exterior of the shield strikes me, for what it is worth, as some of the most effective on the statue, probably because there were detailed extant ancient models.